usually to get better gas mileage there are a few things you can do to help.
inflate the tires to proper or a little higher than specified air. it will wear out the tires a little differently but it will save you gas.
changing tires to thinner tires will help a lot. if your car is running 245 then going 225 or 215 will change thing dramatically.
take out extra baggage in the trunk. things that you know you don't need to drive around with.
using lighter weight oil, where possible. its not just for sound systems anymore.
installing new power and grounding wires. remove all the smaller wires and install 00 or 0 gauge wires for power and ground. the big three as they call it.
larger battery. this will help with the cars efficiency to send more power to the spark plugs and other electrical modules in the car.
doing a tune up. basic tune ups will always help out. change out the air filter every other oil changes. change spark plugs, gap them correctly. etc...
do a dyno tune. this is the biggest help. it will recalibrate the ecu to optimize your car and not only save you gasoline but also the car will perform better. it sounds weird, I know, to get better performance and getter gas mileage at the same time. it really does work.
the last thing I can think of right now is to lose weight. if your heavy set, just go on a diet. this will do the least to help but ehh, thought I would just throw it out there.
these are all the things I have tried myself, plus more, and these are the ones that actually work. (leaving out he last one on the list this just depend on how heavy you personally are).