Frozen scallops

Most people I know who cook scallops actually overcook them. I always buy sushi-grade diver's scallops from my fishmonger. These can be eaten raw, and are most delicious when done so. However if you want to add a little extra flavor, and liven up the appearance of these then sear these briefly on a high heat. If you can get these from a quality source (probably not a supermarket) and you're not hung up on eating raw/rare seafood you'll never look back.

Unless your scallops have been soaked in a brine, and you're buying the quality scallops I recommend you don't need to rinse them. Otherwise, rinse them thoroughly and pat dry with a paper towel.

To sear these properly I simply melt a Tbsp or two of butter in a non-stick pan. I use as high a heat as possible (med-high to high) you want to put the scallops in just as the butter begins to barely smoke. I've often seen people suggest using clarified butter, but I'm too lazy to try. Place your scallops in the pan and cook them for 30-60 seconds per side. Don't move them around in the pan, otherwise they won't sear as nicely. This is for scallops that are the size of a small childs fist. If you have smaller ones you might need to cook them less.

Just a brief update because I feel I didn't stress this enough. Fresh, quality scallops are absolutely not intended to be cooked well done. They will be chewy, period. If you are used to them this way, well you're really missing out.

I've had this issue with crappy supermarket scallops before. Alton Brown covered this. From http://www.goodeatsfanpage.com/Season9/scallops/scallop_trans.htm (section 5):

dry scallops are usually ivory, or slightly pink, or even orange in color, not white. Of course, they don't actually look dry. The term refers to the fact that these lovely lozenges have not been soaked in any kind of chemical, say, sodium tripolyphosphate. This solution is used to help scallops retain moisture when frozen. Now there's nothing wrong with that per se, unless the scallop in question is not going to be frozen. You see, treating fresh scallops with S.T.P. causes them to gain moisture, making them heavier, which could be a good thing for a retailer, but it's never good for a cook. Because once this stuff is inside the scallop, they become very difficult to cook properly, and they are impossible to sear properly.

Diver scallops are probably good. Look for a bit of color when buying. The good news is that you probably aren't doing anything wrong, the scallops just aren't going to sear.

When you buy for searing, get scallops that haven't been treated with chemicals. Diver scallops are almost always untreated. Look for a bit of color, which is usually a sign of quality. Don't buy them if they are in a pool of milky liquid (as they usually are in the fish section of the supermarket). Buying diver scallops is also much more sustainable and eco-friendly, since large ships that freeze their stock at sea dredge the floor to get scallops. Divers are much more selective and do negligible damage to the ocean floor.