My husband got, for lack of a better term, "slack jawed" during chemoradiation, as his tongue was very irritated. In order to help him sleep at night with his mouth closed, I bought him a c-pap strap from the local medical supply store, did the trick perfectly. Once he started using the strap, it helped lessen the mouth sores as well.

Now that he's done with treatment, dealing with dry mouth, has found the strap very helpful in keeping his mouth closed and as moist as possible at night.