In order to play baseball, one needs to know the rules of baseball. Mastering the rules, however, doesn't enable one to play baseball well, to pitch, catch, throw, hit, and run.
Learning grammatical rules is certainly important for writing well, but a mere knowledge of the rules doesn't help one to compose well. Grammar is an essential part, but style and rhetoric are not to be neglected. So, what resources are there for teaching rhetoric and style to students who must compose essays, reports, and research papers?
One resource I found recently seems to help a great deal: Richard F. Nordquist's Writing Exercises: Building, Combining, and Revising. It needs a little adaptation to fit into the coursework of my middle school students, but I find that the exercises are quite practical and attainable.
Richard also has a very helpful grammar blog which I have enjoyed over the years located here: http://grammar.about.com/