Unfortunately some of us have to drive large trucks either because of climate, work, location, or any combination of other things. Most of these large vehicles get very poor mileage, but but with proper care and maintenance we can improve it slightly.
Step 1: Get a topper or cover for your truck
Mythbusters have proved that these aren't the most fuel efficient when the truck is empty, but they didn't try it when it was full of wood or old appliances. A topper can really help when you are hauling bulky unaerodynamic objects by reducing the drag from wind.
Step 2: Reduce weight
This one is pretty obvious. The more junk you have, the more power you need to haul it. Try to reduce all unnecessary weight. For example, get rid of steel racing jacks, and replace then with aluminum ones, or bottle jacks. Also clear out stuff that you don't need like shovels or bags of concrete or stones, and other stuff like that.
Step 3: Re-route air intakes
In the winter (especially in harsh northern climates) you should cover the front grill of your vehicle to increase the temperature of the air that is entering your engine. Blocking the front will force the air to be pulled past the hot metal of your engine resulting in better efficiency. This is especially true with diesel engines.
Step 4: Driving habits
Another simple alternative is to drive more efficiently. Drive at around 55 mph, or the lowest rpm in your highest gear. You should also make sure overdrive is on, which tells your car to shift gears sooner. If you drive a manual, shift up into a higher gear as soon as possible.
When you are approaching a stop light, let the car coast for as long as you can. If you are far away and it is red, sometimes you can time it in a way that you won't even need your brakes. This will greatly increase mileage, considering that it takes the most energy to accelerate form a dead stop.