Good, appropriate formal instruction, with emphasis on those skills as a formalized part of all upper-division courses. I would like to see an undergraduate communications requirement modeled on our graduate program. I try to emphasize the importance of these skills. If so, maybe it should be redesigned. If not, we should.
Smaller classes says it all.
Another solution is to provide a graduate student to work with each professor in order to assist in grading written exams. More courses should require competent written work. Consistency across instructors, at least in a subject area, in grading, emphasis, etc.