What is the best way to clean mussels?
If you mean cleaning in terms of getting rid of the 'beard', use clean pliers and a lot of elbow grease. It's hard to pull that out.

If you mean just cleaning the shells, use a stiff bristled brush and scrub.

If you mean cleaning the sand from the inside of the mussels, put the live mussels in a large container full of water and cornmeal and leave overnight in the refrigerator. The mussels should flush the sand out and replace it with cornmeal (never tried this method, but I've heard it works well.)