Grille blocks (as much as you can)
Air dam (Can do with lawn edging for <$20)
Wheel covers (coroplast cut into circles and zip tied to spokes)
And cover the rear half of the bed, if you're not up to making an aero topper. GM holds a 1984 (?) patent on covering the rear half, claiming it's more efficient than covering the whole thing. By any account, the differences are pretty close. But buying a half sheet of plywood is cheaper!
http://ecomodder.com/forum/member-sv...ture4138-a.jpg