Steven Pinker, who writes both academic prose and trade non-fiction, offers up a field guide on what distinguishes bad academic writing.
Meanwhile Bryan A. Garner, the undisputed guru of modern legal writing style, has his own ten tips of good legal writing. His top ten:
1) Be sure you understand the client's problem.
2) Don't rely exclusively on computer research.
3) Never turn in a preliminary version of a work in progress.
4) Summarize your conclusions up front.
5) Make your summary understandable to outsiders.
6) Don't be too tentative in your conclusions, but don't be too cocksure, either.
7) Strike the right professional tone: natural but not chatty.
8) Master the approved citation form.
9) Cut every unnecessary sentence; then go back through and cut every unnecessary word.
10) Proofread one more time than you think necessary.