This post was originally written (by me) on February 5, 2008. The original is no longer available.
I've recently started running my first D&D campaign for a couple of years, and I think it's going to take a while to back into the extra time and effort that running a campaign takes compared to playing in one. I'm essentially a lazy person, and while there is something to be said for generating an incredibly detailed room-by-room, conversation-by-conversation adventure, I prefer to take a broad plot and then basically make it up as I go along. So the following advice is not for the diligent and meticulous, but rather for those who feel confident enough to wing it. My basic rule is find the balance between flexibility and rigidity, and once the parameters have been set, stick to them. I suppose a post like this should be peppered with humourous images about roleplaying, but I think Shamus over at Twenty Sided has it pretty much covered.
Lie, Cheat and Steal
The player's dice provide as much randomness as you really need in a campaign, so feel free to modify your own dice rolls as necessary. Sometimes it's just not appropriate for a monster to critical hit a player, and it's your job as DM to make sure that the player's are having fun, not too scared of dying to actually go and slay evildoers. And on the flipside, the rest of the players might start getting annoyed if the wizard starts every combat by Power Word Kill-ing the big bad. Don't be afraid to lie about what the dice say.
You will at some point in your time as a DM find yourself in a situation where things are about to get bogged down, or worse have become bogged down. There are a few key indicators that an upcoming combat is going to be slow and boring for the players. One is if the enemies have multiple spellcasters, another is if there a more than three types of enemy in a single combat, and probably if there are more than 20 monsters in a given combat. Obviously mileage may vary depending on the system you're playing with, but too many enemies, too many types of enemy or too complex enemies all require a lot more thought from you, the DM and as such too much time is spent with everyone waiting for the DM to make decisions or roll dice. In combat, the DM basically has two main jobs: tell players whose turn it is, and control the enemies. A set of cards or a small whiteboard can sort out the first one without too much trouble. When it comes to monsters, always remember that players essentially want to know who got attacked and how much damage they took. If you've got an encounter with thirty goblin archers, and they've got about a 25% chance of hitting, roll 2d6 or 2d8 to figure out how many hit, and you've saved 28 dice rolls for effectively the same result. Got three or four mid-level clerics in the encounter, and haven't assiduously written down every spell they've prepared? I generally assume they've got one spell that's perfect for the situation, two at most, and the various general purpose cleric spells, plus cure/inflict. The chances of them surviving long enough to overuse their available spells is minuscule, so don't worry about it.
Players will absolutely destroy your plot, so don't spend too much time making it too detailed at the start. Plagiarise an overarching plot from one of the uncounted fantasy movies, games or novels going around. In my experience, the individual encounters (be they violent or not) tend to stick in the players' minds much more than the reason they got into the situation in the first place. Plus, players tend to like the reassuring nature of a vaguely familiar story.
Be Honest, Fair and Original
Sometimes you need to be a slave to the dice, no matter how much it messes with the players' or your plans. If players think you won't let them die, they'll be reckless and behave even less believably than usual. The occasional player death keeps everyone on their toes. The other side is sometimes powerful enemies will fail a Will or Reflex save and suffer the effects of a death spell care of a PC. Much as it disrupts your adventure, wizards spend their entire time at early levels hiding in corners casting magic missile twice a day and hoping nobody sees them, let them enjoy themselves when the good spells start becoming available.
Players, I find, want flexibility only when it is to their advantage, and strict interpretation only when it is to their advantage. This is hardly surprising, but it can lead to tension from time to time. I find it's a good idea to play magic with as much rigidity as possible: it's magic so trying to argue what "makes sense" is an exercise in futility. The D&D rules have very specific passages about moving in combat and attacks of opportunity. Any time a player is arguing about these rules, it's because they are about to take damage and want to avoid it, so their point has probably got a flaw.
Players are in their comfort zone dealing with a dread necromancer, or being told trinket X can be found in dungeon Y, but they're likely to raise their eyebrows if you tell them they need to meltdown a powerful magic ring forged by Tauron in the fires of Mt Broom. Make sure that the story your group tells (don't ever, ever think that you'll be able to tell your story) has original characters, situations and motives. For every occasion it's useful for players to have a good idea what's going to happen next, there's three where a surprise is better for the game. And never let anybody play as a renegade drow. In fact, even suggesting the character should be considered fair grounds for immediate eviction from the game.