Tools

1. DM's Screen

A DM's screen of some sort is an absolute necessity. Although I recommend the official "store bought" screen, it's easy enough to improvise your own if you'd rather spend your gaming dollars elsewhere. The main purpose of the DM's Screen, of course, is to give you some place to set your various notes and maps without exposing them to the players. Trying to run a session without a DM's Screen is likely to force you into some uncomfortable contortions and slow play -- you won't be able to keep your materials spread out for easy access, and you'll find yourself constantly shuffling through papers. In fact, if you have the space on your gaming table, I recommend setting up two adjoining screens for even more secret space. This configuration lets you simultaneously lay out all your materials and keep a rulebook open to an important page, while still leaving plenty of dice-rolling space.
As you get more and more sessions under your belt, you should strive to evolve your DM's Screen to make it as useful as possible. Do you have a consistent problem recalling a certain rule? Scribble a note to yourself and paperclip it to the inside of your screen. Is your next adventure likely to place the players in situations requiring special rules or charts (such as underwater combat or extraplanar spell casting)? Photocopy the appropriate materials and clip them to the inside of your screen. Similarly, when I'm running a store-bought or prepared adventure, I almost always photocopy short sections of the text and clip them to my screen for reference. If I were running "The Scar" (my adventure from Dungeon Adventures #80), for example, I'd copy the random encounter charts and the sections about sneaking around the complex.

2. Adventure Notes

Some set of papers describing the adventure you are running is another obvious necessity. Unless you are one of the best eight or ten DMs ever to grace the gaming tables, don't even think about running a game without a prepared description of the adventure you are undertaking. A lack of preparation vastly increases the odds that the whole session will collapse into boredom. Sure, you can't possibly cover every contingency in your notes, but a good starting point and a foundation are necessities.
Adventure notes break down into two obvious categories: those you purchase and those you create yourself. The former category is pretty straightforward--you're planning to use whatever the designers give you. (Most store-bought adventures are quite well organized.) When using a store-bought adventure, I always take the time to photocopy any important charts or maps that appear in the text. That way, I can refer to these important materials while keeping the main booklet open to the section describing the players' current activities.
Notes you prepare yourself are another matter altogether. Everyone has his own favorite format, but I prefer to scrawl my notes on carefully numbered loose-leaf paper and keep them in three ring binders. This method allows me to separate the pages so I can refer to several of them simultaneously when necessary. In fact, I've grown so fond of this system that I'll sometimes photocopy smaller store-bought adventures onto individual sheets so I can use them in the same way.

3. Rulebooks

You can't really play the AD&D game without a copy of the rulebooks on hand for reference. One of your goals as DM is to minimize the rules look-ups during play, but a certain amount of page turning is just unavoidable. Always make sure you have your own copies of the Player's Handbook and Dungeon Master Guide on hand, as well as the complete descriptions for any monsters that might turn up during the evening's adventuring -- not to mention any subsidiary rulebooks you're using (such as the various Complete Handbooks). If one or more monsters are likely to turn up in the current adventure, try copying their vital stats onto 3-by-5 index cards before play begins. That way, you can quickly pull out the cards during play and won't need to flip through various rulebooks and magazines.
I use a couple of other special regulations regarding rulebooks. First, I don't allow the players to refer to anything but the Player's Handbook (and the appropriate Complete Handbook) during play. This prevents the players from consulting monster statistics during a battle or referring to the lists of standard magical items when trying to make guesses about newly acquired items. In fact, I strongly discourage the players from bringing copies of the Dungeon Master Guide or other banned books to the session to save space at the gaming table.
Further, I require players with spellcaster characters to keep the appropriate spell descriptions handy during play. If I have a question about a particular spell cast by a player, I expect that player to have the answer on hand. (What's the range on that sleep spell? What's the duration of invisibility 10' radius?) This prevents me from spending a lot of time flipping through the rules myself, and it usually gives me an opportunity to keep play moving by conducting some other action while the player seeks the answer.

4. Character Sheets

While a blank piece of paper can certainly serve as an effective character sheet, I prefer to provide more formal sheets and mandate their use. I go so far as to require the players to fill out the entire sheet before play. That way, I can guarantee that the players are fully prepared and have pre-calculated their THAC0s, saving throws, and various other details. Stopping for a player to look up these numbers during the game is only going to consume valuable time and cut into everybody's fun.
Of course, it's possible to take this philosophy too far. Some of the character sheets I've seen (like those for the 1st edition produced about fifteen years ago) practically require the players to write entire books about their characters. The key is to find a sheet that requires a minimum of fuss but still includes all the important details. The official sheets now sold by Wizards of the Coast are quite good. You can also find some decent character sheets floating around various Internet sites, or you can sit down and draw up your own. I recommend this last course only if you expect your campaign to have some unusual needs. As DM, you already have plenty of work to do -- if you can't find anything else, investing a few bucks in the official sheets is almost always worth it.

5. Dice

Dice are an obvious requirement for playing the AD&D game, though there are a couple of points that are worth mentioning. Don't buy cheap dice made of soft plastic -- the high impact polyhedrons aren't much more expensive and one decent set will last you forever. The softies tend to wear out after a few months. Also, make sure you and your players have enough dice for everybody. Generally, at a minimum this means a full set for the DM, a full set for the players as a whole, and a separate d20 for each individual player. Sharing a single d20 among all the players tends to waste a lot of time and slow down combat. It's also a good idea to ask the players to keep the dice they are using close to themselves and away from the center of the game table. Allowing the dice to clutter up the middle of the table not only makes it easy for players to lose dice they personally own, but it can make specific dice difficult to find when needed.
While it's important to make sure you have enough dice on hand, it's also important not to clutter up the playing area with unneeded dice. For some reason, there are some DMs who like to travel around with huge bags full of the things. I've never understood this impulse. As DM, you already have enough things to carry, and a huge mess of dice is just going to get in your way.

6. Miniatures

Miniatures constitute one of the great controversies among DMs. Some wouldn't think of playing without them; others find them distracting and limiting. Personally, I think they add a great deal to play, and I encourage you to use them every chance you get. That said, there are some obvious drawbacks that prevent many DMs from adopting them.
If you've ever been in a game store, I don't need to tell you that miniatures are expensive. A good starting set can easily run a couple hundred dollars or more. Even after you shell out the cash, you have to find the time to paint the things. The last seventeen columns have probably already clued you into the fact that, as DM, you already have an awful lot of work to do.
Of course, buying and painting miniatures is an enjoyable end unto itself. There's a real satisfaction in capturing just the right expression on that goblin's face or correctly painting chain mail for the first time. Plus, a good collection of painted miniatures is a nice trophy you can use to decorate your game room.
If you're thinking about using miniatures in your games but have yet to make up your mind, here's a recommendation. Never buy more than one or two figures at a time, and buy new figures only when you've finished painting the last ones. That way, if you discover that you don't really have the time or the patience to invest in a collection, you'll minimize your investment and won't end up with an enormous pile of unpainted lead.

7. Battle Maps

Some DMs like to run their combats entirely in the imaginations of the players, taking time each round to describe the relative positions of the various combatants and the surrounding scenery. I prefer to use tokens, markers, or miniatures as a visual aid to depict the action. I find that such a scheme not only saves me time but also encourages the players to be a bit more creative when describing their actions. Showing a player a battle map roughly depicting the scenery inside the dungeon is a great way to encourage him to try an interesting maneuver like yanking a tapestry off a wall down onto the head of his opponent. Another benefit of using a battle map is that you'll find it much easier to adjudicate the range and effect areas of spells, breath weapons, and so forth.
If you're on a tight budget, a pad of paper and a pencil can serve as an effective map. Whenever you start a combat, sketch out a little map of the battleground and place each character's initials on the map to indicate his or her position. As the combatants move, just erase and redraw the initials.
If you've got a little money to spend, an even better method is to purchase a small whiteboard (say 2 feet by 3 feet) and a set of dry erase markers from your neighborhood office supply store. Place the whiteboard in the middle of the gaming table and quickly sketch out the surroundings whenever a combat begins. You can use improvised tokens or miniature figures directly on the board to represent the various combatants or draw in initials as I recommended above.
Like character creation, the new combat rules are home to many of the new edition's innovations. In general, combat is more stringently codified and the various actions the combatants can select are more rigidly defined. It's still possible to run a battle entirely within the imaginations of the participants, but doing so definitely makes it harder to effectively wield all the interesting new maneuvers that the new rules have to offer. Now more than ever, I recommend employing some sort of counters or visual aids to track the positions of characters in battle. Detailed miniatures and scenery are obviously the ideal tools for this purpose, but not everybody has the time or money to invest in building an appropriate collection. In an earlier column, I suggested using a large whiteboard in lieu of miniatures to map out your battles. The idea is that you can quickly draw up battle maps on the board using dry erase markers, and plot the players' positions using makeshift tokens like coins or dice. The board also gives you a convenient way to record the positions and effects of spells and obstacles. Should a wizard cast wall of ice, for instance, you can quickly sketch the ice wall right on the battle map and even record a helpful note right next to it ("36 hp/10 ft., Break DC 27").

8. Notepads

I always make sure to have two small notepads on hand when running a session. I keep one and place the other outside of my DM screen in front of the players. The purpose of the notepads is to allow specific players and myself to exchange secret communications.
Suppose, for instance, that the players are exploring a forest, they've sent their thief up ahead as a scout, and he's just stumbled across something interesting. For now, I might simply pass that player a note describing what he sees instead of blurting out the description to the whole party. After all, if the thief just discovered something dangerous, the rest of the party shouldn't know anything about it unless the thief is close enough to shout a warning.
Similarly, if the thief just discovered some treasure, he might want to help himself to some of the choice pieces before he reveals the rest of the treasure to his friends.

9. Props

I'm a big fan of using relevant props during play, particularly little handouts approximating scrolls, notes and other important clues uncovered by the players. I think that props give the DM an excellent method of subtly communicating important information to the players and therefore lead to much more interesting puzzles (for a more complete discussion of puzzles, see issue #271). Suppose, for instance, that the players are searching a duke's private papers and they find a note the duke received from his lord ordering him to launch a treacherous attack on a nearby town. The fact that a couple of specific words in the letter are misspelled might be a clue meant to convince the players that the note was a forgery. Perhaps the players have already seen another note undeniably written by the king's mischievous brother in which those same two words were misspelled. How can you pass the players these clues without providing them with facsimiles of the notes in question? After all, simply announcing that "You find a note in which two words are misspelled" immediately calls attention to the ruse and spoils the whole thing. This is a great example of the sort of puzzle you should strive to create, and you should try to use such props and facsimiles whenever you can.
On the other hand, I'm not a big fan of using props that don't really advance the adventure or provide the players with important information. I've known DMs and players who like to bring toy daggers, cheap jewelry, or even entire costumes to game sessions to help them "get into character." Personally, I find those sorts of things a little spooky, but if you find that they add to your fun, more power to you.