Slipin Lizard's suggestions are really good. I also find that I rest the back of the neck between my thumb and forefinger unless I feel I have to put my thumb in the back. My left thumb can move back much farther than my right - probably from playing guitar all of these years. Everybody is built differently, so this varies.

Barre chords is where I consistently need to put my thumb in the back and if I play a lot of barre chords, my hand will fatigue.


Anyway...one other thing to look at is how you are positioning the guitar against your body. When you are first learning, you have a tendency to turn the fretboard toward your face so you can better see what your fretting hand is doing. If you are doing this, you are adding to the stress on your wrist by causing it to turn further away from you. The guitar should be parallel to your body. A way to force yourself to not turn the guitar toward you is to play standing up.

Chris S. has some good tutorials on proper body positioning and posture that may be helpful to you.

Good luck!