Eat young fish. Typically, in a given body of water, mercury level in a fish is directly proportional to its age. Avoid, if you like, large fish (e.g. tuna, marlin, swordfish, shark) and small fish which take a long time to mature (orange roughy, grouper, etc), but don't be afraid of all seafood.

Coincidentally, eating small fish is much better for the marine ecosystem as well. They are much more difficult to overfish (not that humans haven't figured out how).

More info on mercury levels in seafood: http://www.fda.gov/Food/FoodSafety/Product-SpecificInformati...