I can't relax my face!

Have you considered Botox?

Could be that you’re thinking too much about it. Try giving yourself a face massage when you feel yourself tensing up. Try jaw exercises, face yoga and smiling. You can look all these up online as to how to do these efficiently.

Have you considered getting hammered? ;-)

Is this a new development? Have you been anxious lately?

Do you wear sunglasses? I squint when I am outside in the bright sunlight. Good sunglasses help.

stop taking ecstasy

Here are some short exercises using mindfulness breathing. I use them all the time.

http://www.getselfhelp.co.uk/mindfulness.htm

Even if you do them for five minutes, your body will thank you.

Meditation is good as people have suggested.
Try yoga facial exercises. They utilize progressive relaxation technique.
If there is something making you anxious try to find a better way of mentally coming to terms with it. Don’t avoid it but face up to it and deal with it as best you can. If there is some stressful situation in your life right now that you can’t change then try not to obsess over it. Put it out of your mind as much as possible once you know you’ve done everything you can to achieve a postive outcome. You aren’t going to do yourself any good by constantly thinking of it (if indeed there is such a stressor in your life, you don’t say there is so I’m just guessing.)
I also like to use sleep machines that produce white noise, or raindrops or the sound of rushing water. That would be helpful to combine with the yoga that @gailcalled suggested and the facial relaxation exercises that I am suggesting.
If you find it hard to consciously relax biofeedback is another technique that can teach you to relax. When you tense a muscle it tells you and you can train yourself to get in a more relaxed state by recognizing the signs of tension in your muscles and learning to let the tension go. You learn how to induce beta waves. It very interesting but I’m sure therapy is not cheap.

They say if you have good muscle tone in your face, it won’t sag as much as you get older.
I think that would be a good thing.
I keep thinking I should do facial muscle exercises to prepare for the eventual scourge of gravity.
Sounds like you’re doing exercises automatically. I wouldn’t try to stop it.