Soak the clams in hot water (45-50℃) for 5-10 minutes. If you have the time, soaking for 20 minutes is best.
Wash by rubbing the clams thoroughly, then discard the hot water. Rub-wash the clams 3-4 times. It's easy to rub-wash while running the clams under warm water from the tap.
And there's no more sand! The clams will open about 3 mm. This is the proof that they've spit out the sand!
Story Behind this Recipe
This method of washing the clams in hot water to remove the sand was shown on TV a while back, then I came up with my own way to do it. It seems that soaking them in hot water forces the clams to spit out the sand. I decided to try washing them at 50℃. The temperature can be changed.
Regarding the temperature of the water... the clams should soak in water at a temperature at about which the meat will poke out a bit (45-50℃). If it's too hot the clams will open up and you'll lose the juices, so be careful.