I soaked them in cold water for a while to drive off the hitchhikers, then carefully rinsed them under cool running water and put them through my salad spinner. Voila!
If you've never had oyster mushrooms (and you enjoy mushrooms) you are missing a real treat. They are the best I've ever eaten. PLEASE do not go out and try to gather these on your own if you are not familiar with them. Get someone knowledgeable to show you first.