More than almost anything else, the quality of fish depends on its freshness. When you buy it, steer clear of prepackaged fish. In fact, the best way to buy fish is to befriend a fisherman -- or at least a reliable, knowledgeable fishmonger, whom you can cajole into cutting fillets from whole fish or steaks to order. Consult with him, flatter him, cherish him. He's the only thing standing between you and a foundering flounder. That said, there are some basic guidelines you can follow yourself, in a pinch.
Buy fish the same day you're planning to eat it. Unlike with steak and wine, there's nothing to be gained with age.
Sniff. Fish, ironically, should not smell "fishy."
Inspect. The skin should have a sheen; the scales should be smooth and undamaged. And the gills of a whole fish should be bright red. Lift them up and take a peek.
Touch. The flesh should be firm and spring back. You don't want to leave a dent.
The colder, the better. Bring a cooler to the market during the summer, and when you get home, stick the fish in the coldest part of the fridge. (Rest assured, the Brooklyn Lager will survive somewhere else.)
Buy in season. Right now, that means monkfish, mackerel, skate, herring, and too many more species to mention. You might want to consult an expert. (It's not like we didn't warn you.) New York is blessed with a surfeit of reputable, high-quality fish stores and fish departments in gourmet stores, and on Long Island, Artie Hoerning of the South Shore Fish Market (4257 Austin Boulevard, Island Park; 516-889-0692) brokers the local catch from day boats. You can't do better than that without your own rod and reel.