Im getting 16 mpg with my '99 F-150 (4x4 reg cab, off-road package) with a 5.4L and 3.73 gearsunder normal driving (60% country backroads, 40% city).
Id try a K&N (it gained me 1 mpg) and check your tire pressure.
Other than that, perhaps try accelerating smoothly and lightening up on your right foot a little bit.
Octane