I’ve just found a brief but very well written Guide to Writing Papers by S. Majid (of quantum groups fame). He makes many a good point, specially for those of us whose mother tongue is not English. And you’ll find also useful advice on contents and organization.
When it comes to writing good English prose, Strunk and White’s classic “The Elements of Style” (also available and searchable online) is, of course, required reading, but i find Dupré’s “BUGS in Writing: A Guide to Debugging Your Prose” much more fun and equally insightful. Also worth a look is the online Researcher’s Bible (hat tip Jocelyn Paine), which is similar in scope to Majid’s tutorial. And for a good, thought-provoking laugh, don’t miss the indispensable How to Write a Scientific Paper, over at Improbable Research.
Let me take the opportunity to recommend two of my all-time favourite essays on scientific writing: the classic How I Write by Bertrand Russell, and the recent A parallel tradition, where Ian MacEwan beautifully makes the case for a scientific literary tradition. So much for the science/humanities divide!
Finally, lest you had not enough, this blog entry by W. Zinsser is, in my opinion, an excellent guide to improving your writing skills (scientific or otherwise), and contains links to several other interesting books on the subject.