A supercharger. Ha, but if you're not rich:

Start with a K&N air filter. It won't make a big difference (you probably won't be able to feel it) but it starts freeing up the air flow. $50.

Then I would put on a cat-back exhaust system. These replace all the piping and the orginal muffler with a high flow muffler. This frees up the air flow some more. $220-$460 + installation which you can do yourself.

Headers. These things can be a pain in the ##### to install. But they free up air flow and really do help. $300-600

Ignition system. This creates a bigger, better spark in the cylinder. This makes for more efficient fuel burning. $200-250

Ilegael (spelling) but maybe you don't have to worry about it. Get rid of the cat or punch it out (scrape out all the insides). Ford's cats flow fairly well to begin with and this really isn't nessacary unless you really want to do it.

At this point you probably want to do something else to your intake to free it up more. Throttle body spacers or a different intake.

You can also do things like chips(requires premium fuel), underdrive pulleys (wears out what ever part is being underdriven), electric fans (wears out the alternator but might be worth looking at).

Nitrous, Superchargers, Turbochargers: All expensive but they work extremely well.

Also, a good tune up will help.

These are external engine mods, if you want to get internal I would say port the exhaust valves maybe the intakes to and get a slightly larger lift cam. But, I don't know much about internal engine building.