Keep it well maintained
Remove the roof rack
Keep your wheels light (stock alloys and 4 season radials as opposed to AT/MTs)
High octane fuel
powerchip
Generally, for fuel economy to go up you need to stay light on the gas and at the same time make your engine more efficient. Better camshaft, better heads, better exhaust, etc.
Get a second more fuel efficient car, ride a bike, walk, carpool, or take the bus if you want to save the environment (seriously). If you want to save money, you aren't likely to be able to make up in gas savings what you spend in modifications.