I found this document based on IEEE-829this document based on IEEE-829 and have used it for rough guidance, but remove categories that seem like overkill for my team or merge categories that seem similar. Since I started doing this, I've received many compliments on the clarity of my test plans. I use a wiki for documentation, which makes it easy for users to jump to the sections they care about.
UPDATED: Previous link no longer exists, so switched to a similar resource.