Yeah, The spark plugs would help, I went and got platinum Spark plugs, Which I scored for next to nothing off a friend, And I will never look back! Before that I used spark plugs with copper insulation and what not, They worked great as well.
Chang your Fuel filter, And don't bother changing your oil filter, do that every time you service it. And I noticed with my Holden Commodore, that it was more fuel economical Using Premium Unleaded instead of normal unleaded, If you don't wish to use premium just use some normal un-leaded and run some injector cleaner through it.
Octane booster also helps save fuel, As you don't have to accelerate hard to get to the required speed, Which in turn saves fuel.
I get a huge bottle of octane booster donated to me, to use on the burnout car that me and a few mates built, Not to mention free Fuel thanks to our sponser
Also I'm not aware of the prices of LPG Gas over in the states, but would it be worth getting LPG put onto your car? You loose a bit of economy, And power, But its soooo cheap its like most of the time only half the price of petrol or even a third, In Australia.