I'm starting to get less gas mileage...?

Ok, so I'm going to put it simple. What are the top 5 reasons my 2002 civic is starting (or at least is appearing) to get less gas mileage. I believe the car was rated at 32/38 but now it seems like every time i put 10 bux in, I get about 5 bux worth, seriously like 22/28. its really starting to suck. And i don't think it's my imagination. What could be the problem here? Air filter, spark plugs, tires, etc. And i'm not gonna go get it diagnosed by some honda guy, either. any help will be much appreciated!!

thanks guys

car has under 55,000 if that matters.