Jonathan Drain, who writes Jonathan Drain’s d20 Source [1], has pointed out a clever way to get free tips on writing adventures: from Dungeon Magazine‘s Submission Guidelines [2] (168kb PDF).
Their guidelines are full of tips on what works, what doesn’t work and what’s been done to death. Although they’re obviously geared for D&D, much of what they have to say can be applied to other games, too.
(Tips on Writing Adventures [3] and Scenario Creation Basics [4] here on TT might also prove useful.)