Dentists make silicone night guards to prevent this, but wearing them regularly can be tedious. There is a much simpler solution.
Botox, useful in so many areas, helps here too. Applied to the jaw muscles in the right dose it relaxes them while still allowing voluntary chewing, and limits the involuntary clenching during sleep. In short, it prevents the involuntary contraction of the powerful jaw muscles. Repeated a few times at 3 to 5 month intervals, night-time clenching can be prevented permanently.