Drive when theres no traffic and go easy on the gas pedal.
This is what I acheived doing so this morning at 3:00 am!!
And note that this is with 20x12 rims, 37'' tires and a 12'' lift.
I do have a pretty nice exhaust setup which did help a little bit and a volant intake.
But really though, its a truck. You cant expect much out of a truck for gas mileage unless you got a diesel with expensive mods added on.