Sette Rim Tape
Every rim needs tape and sometimes replacing the rim tape can cure those mystery flats. Sette Rim Tape is much better than plastic or rubber for preventing pinch flats so next time you replace your tubes try putting some Sette Rim Tape on there as well. Enough tape for a single rim.
This tape works as well as big name brand rim tape. I've also cut small pieces of this rim tape to patch a clincher tire after being punctured by a big nail.