Charcoal canister is a good bet if you like to top off at the gas pump . . which I learned you shouldn't do. Another place to look is the seal around the fuel pump. Rip out the back seat to check. Lastly, check your fuel injectors. If you're getting bad as mileage and the car is hard to start when cold, you may have leaky injectors. The fuel lines themselves are not likely to be leaking, since the crud in the fuel helps to seal the joints and connections.