Heer are our guidelines:
Choosing mussels: Mussels are sold in baskets or containers, by the liter or by weight. In a restaurant, you often get a whole kilo as a portion, but by the time you've de-shelled them you end up eating 300-400g. Someone with a small appetite will manage around 1 liter; someone with a hearty appetite may get through 2-3 liters!
If you buy your mussels fresh, make sure they're alive, the shells are closed, they're heavy (but not too heavy, which could mean they're full of mud!) and bright in color. They keep in the fridge for up to 3 days in a container covered with a damp cloth. They can be frozen, but should be used within 3 months.
Once you've rinsed and soaked your mussels a little, throw out any shells that are slightly open, which could give you a nasty case of indigestion!
Making mussels tender: Scrub, wash and drain them, place in a basin and add a few handfuls of flour. Mix and set aside for 1-2 hours, then rinse the mussels and cook according to your recipe. This is the Belgian way of making them soft and tender.
Frying mussels: Dip them in light batter made from flour, sparkling water and beaten egg white and fry them in hot oil for 30 seconds.
Once you've cooked them dry (in a big pan, until they've opened up), you can also marinade them in spicy tomato sauce, garlic vinaigrette or flavored olive oil.