#1 Slow down, under 2000 is a must, the lower the better
#2 Make sure the front hubs are unlocked
#3 Make sure the air conditioning isn't on, Face/Off/Floor is the only positions it's not on.
#4 No lifts, No big tires, No stuff hanging from the truck
#5 Get the truck warm ASAP, you are warming 15 quarts of oil, and the Exhaust Back Pressure Valve cost about 3 MPG when in operation.
#6 No idling, Idling equals zero MPG.
#7 No having fun, accelerating an 8000 pound truck isn't good for mileage. Anticipate the lights etc, so you don't have to slow down.

Other than making sure the engine is running properly that should cover it.