Just read this IBM developerWorks article about the poor state of documentation. Fair enough, but whining about it without providing more specific examples, for real-world products, isn’t tremendously helpful. (I rarely write documentation, but I have done some for internal tools.) The only tip to improve the process was on indexing.
Want to make a good index? Ask people to look things up in it, but require them to write down the words they think of before they look in the index. Now make sure all those words are in your index. You’ll be stunned at the gaps this uncovers.