Teacher Magazine posted this fine article today by Cindi Rigsbee about how to teacher grammar in this day of IM and pop culture. She makes some very sound suggestions, like mini-lessons and connecting to students in their actual experience.
One valuable point she makes is to distinguish the levels of rhetoric. In classical rhetoric they spoke of the high and the low style. We speak today of the formal and the casual. By allowing for the casual style, you enable them to allow for the more formal (really the middle style) of academic writing.