http://ecomodder.com/forum/EM-hypermiling-driving-tips-ecodriving.php
read this. your driving style can make the biggest improvement in mpgs over anything else.
99% of the mods you can do to save money would be better off spent as fuel in the tank. if your not doing it to save money on gas, but rather a better running more efficient truck, there are a couple of things.
#1 most helpful mod will be a custom tune done by any number of companies/people that do them. if there is a reputable shop nearby thats always best as it makes it easy for re-tunes. justin at black bear performance has a great reputation on the forums. while your at it, have it tuned for the 93 or 91 octane whichever is more available for you, and you will see even better gains. you will pay a little bit more at the pump but you will get it back in mpgs if you can keep your foot out of it.
everything else will be in conjunction with the tune. cold air intake (air raid jr has been tested by black bear as the best performing CAI on the market. and it can be had for around $100 all day)
exhaust (long tubes and a magnaflow have great results. magnaflow makes an aftermarket cat if your like me and hate the smell of rich exhaust or you can just have your factory ones rewelded)
everything else will be things you can find here
