For myself, the two biggest mpg gains came with modifying my driving style, and modifying my route. A lot of my other mods are just fluff compared to those two. But when you add them all up they collectively make quite a difference.
Of course, buying myself the highest mpg hybrid ever built with my gas savings hasn't hurt the cause either
My favorite mod is my three-button shift knob, though. That's one badasss mod right there!