Put the shijimi in a bowl, and add some water and an iron (or iron-coated) utensil.
In the summer, it's better to put them in the fridge to de-sand. If you are freezing them, de-sand them and wash the shells, then place in a zip bag.
Story Behind this Recipe
This is my mother's secret trick!
Shijimi live in lakes, so it's up to you whether you add salt or not. (Note: If you are using shijimi from brackish waters such as Lake Shinji, it's fine to add 1% salt.) Just adding an iron utensil to the bowl gets rid of the sand very quickly.