That's pretty typical mpg. Most of the modifications you can make to increase mpg are pretty expensive, and not going to save you any money in the long run. One that is less expensive is a more free-flowing intake (and you can balance that with a larger exhaust, but that will add cost and be louder) Most of the changes you can make are in your driving style.
You can put a tonneau cover on the bed to decrease wind resistance.
Take it easy on the gas pedal, try to ease into acceleration slowly, and try to ensure shifts occur below 2000 rpm. If you have a manual transmission, shift below 2000 rpm.
Anticipate stops- if you suspect a light is going to change, let off the gas as you approach and coast. You may still be rolling when the light turns green. Stopping and starting from a stop uses gas.
Shift into neutral on downhills.
If you sit at stop lights or railroad crossings, shut off the engine.
Check out***@******.***. People register their vehicle there and track their mpg. 14 is the most common, but it is not broken down by year so newer trucks may be more efficient.