How to get better gas mileage out of a 99 Nissan Sentra GXE limited edition?

From what i have heard my nissan sentra is suppose to get around 33-36 MPG on the highway. Well my car usually only gets around 28 miles to the gallon right now, and i have been trying out the lucas fuel injector cleaner, its seems to work for about 2 days then its goes to bad gas mileage again. Also i think i have to take into consideration is that it also is near 220,000 to 230,000 miles on it. can anybody give me tips please.|||It's supposed to get that mileage when new, after 220,000 miles, 28 it probably good.