For good gas mileage, you are supposed to draft. Mythbusters did a test on it a while ago, and the results were pretty good. However, as a truck goes up a hill, it is unlikely that you are willing to wait, or if the savings is truly worth it. So you ever draft?
I try to, but do not go out of my way to do so. I don't ride on the bumper either; I just stay about 100 ft behind, using the 2 second rule. I think it gives me about 2 extra mpgs, and I can feel the difference. What about you?