Two things: One, yawn as much as possible, whenever you are at a stoplight, waiting in line, whatever. Yawning helps relax muscles that you are straining.

Second, as said above, make sure to warm up. Start in the middle of your range (the most comfortable part, where you like to sing) and sing a few notes down, back through the middle, and a few notes up. Then do it again, adding a few more notes -- just a few. Keep crossing through the middle of your range and go back there any time you get tight. It's sort of like stretching a rubberband. Also, exercising your lower range first will strengthen your high notes. It sounds funny, but it's true.

Good luck