These days I buy my Mussels in 1.4 kilo vacuum packs. Not only are they all perfect, but they are all guaranteed live. They cost over twice the price of loose ones, but it's worth the extra cost.
I finely chop some onion and garlic, fry in butter and olive oil, then add a good splash of white wine. When all is bubbling away I add the washed Mussels and wait for about 5 minutes. All very classic.
Lady Magnon suggested that I add some cream, but I told her that too much luxury wasn't good for her.
A very pleasant, and cheap, lunch. We should eat them more often.