M.L.
Keep them fastened when you're not using them. If you don't have a hanging device to store them (long ago my mother made me one out of an old rolling pin with cup hooks attached), you need to buy some of the small bags you'll find in the jewelry-making department of the craft store, and keep one necklace in each bag. If they can't get near each other they can't tangle themselves up. (Or can they?...)