Julia says to scrub them throughly, pull off the beards with a small knife. Then soak them in cold
fresh water for an hour or two so they will disgorge their sand. If you want to add flour on the theory that they will fatten up and disgorge more sand than plain water, the proportion is 1/3 cup flour per quart. Beat the water into the flour slowly at first so it dissolves thoroughly.
I've read to discard any that are open, but if you tap an open one and it closes tightly in response, it's OK.
No matter what you do there will always be some sand in the bottom of the pot. Just decant the liquid (the best part) carefully.
Now if you'll excuse me I'm off to Mussel Beach to flex my abductors.