Aside from actually playing role-playing games, one of my favorite things to do is to read various articles and sourcebooks with useful game-mastering advice. I'm always looking for more advice from different sources; So, what's your favorite piece of published GM advice ?
The first of mine is a little gem of a weekly e-zine called Role Playing Tips Weekly. This 'zine is dedicated to the fine art of game-mastering and is full of useful advice each issue. And the fact that its free makes it even better. Even Gary Gygax is a subscriber. I'd recommend also buying their CD of back issues (helps support the web site and e-zine) but I'd wait until their disc is updated to include all 200 issues.
Another book I've found extremely useful is Robin's Laws Of Good GameMastering by (who else ?) Robin Laws (Designer of Feng Shui; Hero Wars.)
Target your player types; Analyze your RPGs and choose a rules set that suits them best; Tailor a campaign to their playing styles. And, above all, learn how to stream-line your GMing when behind the screen.
At first, it may seem expensive ($9.95 for 32 pages); But, its all text except for a few charts and is some of the best GMing advice available. It was an Origins Award nominee for Best Game Aid or Accessory of 2002. If you read it, you'll know why. Well worth the money.
Anyway, what's your favorite piece of GMing advice ?