04 silverado 1500 MPG pain?

wondering if anyone out there with automotive experience would have any suggestions on things that i can do to increase my MPG. i keep the air filter clean. do things like sea foam and other additives work? would getting the injectors cleaned or the fuel filter changed(would do it myself but its in the tank), new plugs, things of that nature be worth the money spent? im realistic on what i can get out of a 4 year old V8 but i know i can better the 13mpg its getting now.