Good grief!
I've been working on the rulebook for my game, and I found that it was way, way too long. Once I was on page 7 and counting, I scrapped it and started over. I now have it on two. Throwing out what I had was the best thing I did for my game:
Things I've found, that may be helpful to others:
1) If you use humor in your rulebooks, only use it in the introduction and SPARINGLY throughout the rest of the document. Do not litter each concept with a humorous anecdote.
2) Keep your writing clear and technical.
3) Define terms either before you use them (pre-glossary) or as you use them (inline-glossary), rather than saying 'this will be discussed under such and such section.'
4) Unify your rules. If your rules are simple, your rulebook will be simple (much of my game changed during the rulebook writing step... and mine is mostly a card game).
5) Examples, examples, examples. "Show, not explain" - think about how you explain your game to others verbally: you use examples of gameplay. Do the same in your rules to illustrate complex concepts.
6) Avoid exceptions to the rules. Your rulebook shouldn't have a bunch of 'except' statements in it. Standardize EVERYTHING.
7) Let someone else read it. This should be separate from playtesting. If they don't understand something, you probably need to clarify it.
