programmer (that uses custom tunes like a gryphon) CAI, Exhaust, bed cover (topper or flat) smaller tires all CAN help your fuel economy and power to boot... but learning to keep your foot out of it helps the most
time your drive right so you always get green lights so you dont have to slow down. or if you see the light change coast to the light and time it just right so by the time you reach it, it will turn green.
Driving style is the biggest factor. Ive improved my mileage by dropping my speed on the highways a few MPH, coasting to stops as much as I can and with my tuner ive lowered the RPM that the TC locks which keeps the truck at a lower RPM when cruising at lower speeds. Ive gained an average of about 2 MPG by doing all that. Also its important that your truck is up to date with maintenance and tunes ups as well as having the appropriate air pressure in your tires.
What the others posted about keeping your foot off the pedal is key.
I too once wanted great gas mileage, but I decided I wanted to enjoy my truck instead of driving like a maw maw all of the time just to get 16.5mpg
Much more fun driving however I'd like and getting 14.5-15mpg
But, I have an 00 civic for sale if you really want to save on mpg! =)