Good highway tires, control your lead foot, not too much junk in the trunk.
All in all, after 6 years my Trac gets 8-11MPG City and 16-22MPG Highway. Nothing is going to make it jump up and get 30+MPG. Even my friend's Oddysey, which fires on 3 cylinders cruising, gets 26-27MPG unless he's on a long road trip.
I think your money is better spent on good maintenance of your Trac so that it always stays in the proper operating specs, and gives you a good long life so you get your money's worth.
The only snake oil I swear by is a bottle of Dura Lube fuel system cleaner just before every other oil change, and high quality gas from reputable stations.