How to stop clenching my jaw?

Lately I’ve been getting a lot of headaches, and then I notice I’m clenching my jaw kind of hard, which I didn’t realize I was doing until I was already in pain. I’m not sure if the headaches are being caused by it, but that’s mostly when I notice I’m doing it.

I keep saying “okay, I’m going to remember to relax my face” and then an hour later, I realize I’m doing it again.

I don’t think it’s stress, since I just noticed it and I’m Fluthering and watching the Joy of Painting.

How do you stop something you’re not even doing consciously? Maybe something akin to solving the issue of someone grinding their teeth?