For the first time, someone in the real world has recommended that I buy Eugene Volokh's Academic Legal Writing, recently released in a second edition. Amazon sells it for $20.50, and the positive reviews on which site include recommendations from IsThatLegal.org and co-blogger Dennis N. Morgan.
Cheap alternatives abound: the professor also has printouts of Volokh's PowerPoint summary, and there's always Volokh's "Writing to Think Like a Lawyer" contribution to the De Novo symposium.
UPDATE: OK, this professor clearly reads blogs. The writing assignment nicks its opposing arguments from the Becker-Posner blog. Question: Is it cheating to look at these materials?