Have the oxygen sensors checked out (in the exhaust system). These report to the car's computer which controls fuel flow in the engine and if one (or more if multiple) goes bad then it can seriously affect gas mileage by putting more fuel than necessary into the engine. Possible Mass Air Flow Sensor issue, anyone?
Does the car feel like it decellerates any quicker than usual when you take your foot off the gas? This would indicate some sort of drag, sticking brakes, wheel bearing issues? Jack up the car and see if the tires spin freely, remember to use wheel chocks on both tires the opposite side since you need the parking brake off (and manual trans. in neutral). Wiggle tires to see if there is any side to side play indicating worn wheel bearings.
Is your tire pressure up to snuff? Underinflated tires can affect fuel mileage. A tire can be as much as 10 psi low and not visually look much different than a properly inflated tire. Get a good air gauge (one with a dial, $7+ or so at auto parts store).
-T
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