It's kind of ironic, for me, that Johnn Four picked Game Master Mistakes as the topic of this month's RPG Blog Carnival. Roleplaying Tips and Campaign Mastery are both great resources, and his GMing advice is, in general, solid. But Roleplaying Tips was also a major part of my most important game mastering mistake.
See, I've got this perfectionist streak. It's under control now, but it used to be a pretty vicious one. There was a period in high school when I would fail entire classes from simply not doing the work, and while that can't all be laid on that tendency, a big part of it was just that my idea of what would be "good enough" was so overwhelming that I didn't want to start at all. Naturally, that was about the same time I started game mastering.
So I went through this stage where I was obsessed with game mastering advice. I pored over the 3e DMG, read the "running the game" section in every roleplaying book I could get my hands on, and combed the net for articles on technique and style. That's where Roleplaying Tips comes in: at the time, it was the major source of game mastering advice on the web, at least that I could find.
And despite all that, the games I ran were terrible. The advice had nothing to do with it; though the quality varied depending on the source, on the whole it was fairly good. But I put more effort into "doing it right" than into just running a fun game. It took me a long time to figure out that it didn't matter how nicely my notes were organized if there was nothing for the players to do.