Soother!
just kidding lol. I'd like to know this as well. I've had it so bad once that when I woke up next day, my front palate was swollen like a balloon. Hurt =(
Magnesium can help with muscle spasms and cramps, so it could also help with the muscle tension that causes jaw clenching. Although for the magnesium supplement to have any real effect you'll need to be taking it continuously for a few weeks.
if you eat too much magnesium you might shit your guts out though, keep it under 450mg, and try to take it the day before, aswell as the day of rolling
neo, i find if you take it at least a hour before it does have a noticeable reduction on clenching