When you get the mussels from the market make sure the bag they are in has some little holes so that oxygen circulates, and get an extra bag of ice to keep them cold until you get home, then put them straight into the refrigerator. Only buy them on the day that you plan to make them, you want them as fresh as possible.
Before prepping the mussels, discard any that have cracks or holes or are not tightly closed. Then soak them in a colander in a big bowl of fresh water to get any sand out for 15 minutes. Next, lift out the colander and give them a quick inspection. Pull out any beards- long threadlike bits near where the shells meet, and brush off any bits of seaweed or barnacles on the outside of the shells. Give them a final rinse under cold water then add them to the broth and move them around so they are all slightly immersed. Sprinkle with the parsley and leave to cook for a few minutes, until they are all opened (one or two may remain closed- discard these ones).
Recipe by Love from the Land at https://www.lovefromtheland.com/white-wine-mussels/