Create salt water that's almost as saline as seawater. Use salt made out of seawater instead of rock salt. The ratio is 30 g of salt to a liter of water. Remove clams that have cracked or chipped shells.

Spread the clams on something like a shallow container with sieve in a way that the clams don't overlap. Completely submerge the clams under the salt water. Ideally, the clam's tongue will be above water when it protrudes.

Work in an area that's as dark as possible. I covered them with newspapers. Please note that the clams will spit water.

If you look in carefully, you can see their tongues (or feelers) poking out from the opened shells. Let it sit for 4 hours~overnight.

Once you remove the clams from the salt water, wash off any dirt on the surface by rubbing the clams against each other.

Drain and let the clams sit for an hour. This is done to remove any salt. The calms will spit any salt water. Your food will become salty without performing this process.

Wash them lightly, and freeze in Ziploc. It will last for a month. Some people say it'll be okay for half a year.

If you're making miso soup, put the clams (while they're still frozen) into boiling hot water. Add some miso as soon as the shells open and you can make this into a clam soup. The same thing goes for sautéing this with butter or steaming it with wine; i.e. add the clams to your pot while they're still frozen.

It is said that the sand/dirt inside the shells can be cleansed due to the fact that the clam moves around after opening its shells (as opposed to it actually spitting dirt), so make sure the clams don't overlap in the container during this process.

Note: If you don't freeze the clams right away, the tongue can come out a bit and thus will freeze that way. It may surprise you a little bit but it's still edible.