More Power

I have a 1998 Expedition with the 4.6 liter engine. I already have a K&N Air Intake and a Flowmaster muffler. What are the biggest things I can do to get more power, without buying a supercharger or something like that. I want to buy some new headers, and I want other things along those lines. Any help would be greatly appreciated.

I have a 99 4.6 i have added a 75mm throttle body by mack spacer by arid flowmaster 40 series duel pipes K&N cold air and what brought it all together is the hypertech power programer try this site out http://www.rpmoutlet.com/newf150.htm recently i purchased a g-meter and rated my truck at 281 horses at 71mph and have done a 1/4 in as fast as 16sec next I am going to add shorty headers and a pully system check back with me later and I will let you know how it works out