Singing requires a looser, more flexible jaw than most of us naturally have, and just like your hamstrings, it's good to spend some time everyday helping those muscles find freer ways to be. Two or three minutes can be enough. Here are some things that I have tried that work for me. They are in no particular order. If something doesn't feel good, don't do it.
1. Hum a simple song on an “ng” sound, and move your jaw up and down, in a chewing motion. “Ng” is made entirely with the back of the tongue, so the jaw is completely free to move. This may feel strange at first, because our speaking habits tell us that the jaw and the tongue must always move together, but this isn’t actually the case. Notice how tall you can open your mouth while singing on “ng.” To find the sound I mean, say the word “sing” and hold the last sound – that’s the nasal vowel “ng.” If you plug your nose, the sound should stop completely. Feel free to make any collection of pitches that entertain you. Notice what it's like when the jaw moves and the tongue doesn't.
2. Place your fingers gently on either side of your face, near the jaw hinge, and open and close your mouth. Watch yourself in a mirror: is the motion smooth? does the jaw move straight up and down, or does it go to the side a little? can you freely move your lower jaw in a side-to-side circle, as a camel chews? in both directions? Go slowly. Notice how smoothly your jaw can move. The muscles might feel tired sooner than you think. Try to stop before that happens. Lightly massaging the jaw hinge might help it to loosen up.
3. Place one tennis ball in a tube sock and take it to bed with you. Lying on your side, without a pillow, find a comfortable place for the tennis ball to nestle near your jaw hinge, on the outside of your body. You’re lying on your side with a tennis ball inside a sock under your head, in a place where the gentle pressure of it encourages the jaw to loosen. Please don’t try this on the floor – the floor is too hard. First spend 30 seconds here, and notice what you notice. Switch to the other side. Is one side tighter than the other? can you relax around the tennis ball?
The friend who told me about this starts every day this way. Before she even gets out of bed, she lies on a tennis ball. It’s made a huge difference in her quality of life.
4. Put another tennis ball in that same sock and lie on your back, positioning the two balls on either side of your spine, at the base of your occiput (the bony part of your skull that starts where your spine ends). I like doing this on the floor, because it gives me a deeper release in the back of my neck. I find that loosening the back of my neck also brings more spaciousness to my jaw. Try it in bed and on the floor and see what works for you. It might help to tie a knot in the tube sock to keep the balls from slipping out.
5. Last, but not least, sing the opening line of "Deck the Hall" only on the syllable "vo." Freely open and close your mouth each time to make the consonant "v." You might need to sing the song slower than usual (the fa-la-la's are pretty challenging when they're sung as vo-vo-vo's). Expect your jaw to be more willing to move the more times you try this. Sing in a range that's comfortable for you. Let the jaw feel active, but not clenched. Aim for puppet-y floppy. What's it like?