All of these guys have provided good information. As was stated, the best way to improve mileage is to go easy on the throttle as much as possible. I am very easy on my truck. I drive about 25 miles one way to work every day on rolling country roads and some minimal highway time. I see about 19-20mpg's typically, but have gotten nearly 24 on some longer highway trips. On the flip side, I have seen as low as 17 on a tank where there were some shorter trips and lots of starts and stops.
As far as the lightweight tonneau cover goes, I bought one last fall and have seen no improvement in mileage at all. The whole thing only weighs about 20-25lbs. or so. I mainly wanted it for its functionality and to keep the bed fairly clean. However, like I said, it didn't do anything for my mileage. I bought it used for only $125, so it was certainly worth it, but they go for $450-800 new. It would take you the better part of your life to make that up even with a 1mpg increase.
