Using random pieces in a scrappy can be quite liberating. If the colors go together in the pile, they will go together in the quilt. If you can't give up that much control, presort them into values (dark - medium - light) like has already been suggested, then pull from those piles. I've seen the rule that you can ONLY exchange if you pull the same piece as you just had. Other than that, use what you pull and trust the process. It is so much fun.