most of the mods out there claim to increase your mpg, but some aren't exactly cheap. Air intakes and throttle spaces are probably the cheapest. And then there is always exhaust and headers and programming. Like I said, not the cheapest things out there. Just keep the gas pedal off the floor, that should help.
2003 Pewter H2 with Adventure Package, 20% tinted front windows, K&N aircharger filter kit, 3 screen DVD and Playstation 2 system. Soon to come: 37" MT/R's, Warn winch, XM and subwoofers.