Break things down to short sentences. If you're having trouble writing a sentence clearly, break it into two or three shorter sentences. People get scared and don't read them. Magazines will go so far as to take a quote from the article and print it, in the middle of the page, in a giant font, just to avoid the appearance of a full page of text. Magazines will go so far as to take a quote from the article and print it, in the middle of the page, in a giant font, just to avoid the appearance of a full page of text.
Nothing improves a spec more than lots and lots of screenshots.
A picture can be worth a thousand words. Anyone who writes specs for Windows software should invest in a copy of Visual Basic, and learn to use it at least well enough to create mockups of the screens. Um, well, I was originally planning to have a lengthy exegesis of this rule here, but this rule is just too simple and obvious. When you find a sentence that isn't super easy to understand, rewrite it. I've saved so much time by not explaining Rule 4 that I'm going to add another rule.