That's the million dollar question...
Well, gears are a popular mod. They don't give you any extra power, but they give you a 'kick in the pants' feel. If you have a manual do some 355's or 373's, and if you have an auto go no lower(numerically) than 373's.
To pull some serious power, you will have to build the engine; or just put on a supercharger lol.
While they won't set the world on fire, little things like new plugs and wires, cap and rotor, and a timing increase can get you some more power.
Then there are the bolt-on's like a cold-air, pulley's, high flow or off-road cataltic pipe, etc. Do a search and you should find more information.
Good luck.