Low mpg/gas mileage

I have a 99 Accord Lx and for the past couple months I have been noticing a decrease in my mpg. I was getting around 25 mpg every tank just doing city driving. After a while i noticed that it kept going down gradually... first to 23 mpg, then 22, 20, 19, and now 18.5 where it's stayed constant for about 5 tanks, which is about 2 and a half months i would say. It's been really pissing me off, especially because of the gas prices but i've checked my brakes to see if they were dragging, and replaced them, checked my air filter and that was fine, i know i have to check the fuel filter and pump to see if its been clogged and i dont believe any spark plugs are bad. Does anyone else have this problem, or know what else to check or how to fix it? Any help would be greatly appreciated, thanks in advance.

click this link http://www.automotiveforums.com/vbulletin/showthread.php?t=348605 or check out "Accord Coupe Gas Tank Range" post in this forum... we are discussing the exact same thing.

How many miles since the plugs were changed?
Might need to put it on a smog machine to see if you are getting high hydrocarbons and/or CO.