This little guide is a simple attempt to improve the quality of your sessions. I have not included things that are obvious or well-known, or included in every Gamemaster's Guide on the market. If you feel that something worth saying has been left out, or if you disagree with something I say, feel free to drop any comments at firstname.lastname@example.org.
Leave the NPCs in town.
NPCs in the party will bog down conversations and battles, giving the players less time to act with their own characters. Keeping one in the party as your own character is fine, but try not to hog the spotlight.
If they don't know X, they won't care about X.
Don't expect the players to get emotional over somebody they haven't spent time with. If a character or place is important to your story, give the players plenty of time to get to know them.
Follow the rules.
You have infinite power. Use it with discretion. If the dice overrule your plans, then so be it. You can always make more plans.
Vengeance is yours, but the stayed hand wins the heart.
Gamemaster's revenge has no place in a mature gaming environment. Nobody wants to play with a despot.
Hide the strings.
At all costs, maintain the illusion of your players having free will. They don't like to feel like they are being coerced, forced, cornered, or "nudged."
Play to the mood of the group.
If your players are bored, liven things up, right away. This is especially needed in long battles.
Resources don't matter.
There are more important things to worry about than water, rations, ammunition, and carrying capacity. Sometimes you have to ditch the numbers and just guesstimate before the game gets bogged down by trivial concerns.
And encourage your players to do the same. Spending time out of game to create a quick-reference sheet is better than hunting down information in-game.
Don't plan everything.
If the storytelling is as interactive as it should be, the players should have you flying by the seat of your pants.
Acts of God should never happen.
Stay away from strange and random events, even if they are believable within the limits of your story.
Tailor your adventure to the party.
Give the players ample opportunity to use any special abilities or traits that they may have. Never try to "work around" a character's special ability.
Don't repeat yourself.
The party should never meet two NPCs with the same personality. They should never fight the same battle twice. Every castle has a different look and feel. Every mission has a different goal and a different path to get there. A campaign that stays fresh and original is worth coming back to week after week.