Lookin' to do something like this?
http://www.fokisd.org/gallery2/main.php?g2_itemId=508
I have a banjo-rigged "fastback" fairing that I am looking to convert into a more usable topper this summer. For me, the "fastback" has been worth 3 MPG improvement over the open bed.
I certainly can't quibble with basjoos' success, but for me I've gone with the air dam. There are just too many places under the truck I need to get to so I minimize air flow under the truck with an air dam. Actually the air dam was removed for the winter, and I am making an "Air Dam 2.0" that will be more rounded in the plan view. also side skirts are coming.
IMO, reducing weight is not a big help to pickup MPG because a truck is a truck and it has to haul stuff. If you shave off weight, it just comes back as payload.