If the truck is new, the mileage will suck. As it breaks in, it will improve. Mine was lousy until about 5000 miles.
Beyond that, keep the tires inflated and leave your ego and testosterone at home when you go somewhere. You don't have to beat everyone out of the light. You don't have to pass everyone. Pretend you're driving a stickshift and look far ahead and try to anticipate lights so you come to a complete stop as seldom as possible, On long trips, use the cruise control and sit in the right lane unless it's full of semis going below the speed limit, when you're allowed to pass. Otherwise let the BMWs and rice rockets pass you.
Hard acceleration kills fuel economy. Idling kills it really bad with the hemi. And above 70 mph economy falls off really really bad.