Originally posted by Led Zeppelin
Well the pad of your thumb should be behind the fretboard, if you want good technique you should always play like that, directly opposite your middle finger at all times

Use your thumb behind the neck to "press" for some leverage. You can also bring your fretting arms elbow into the your body so that it is about a 45 degree angle with the fretboard giving you more leverage to press down. This technique becomes void with those who can play barre because they are strong enough in an all or none concept. This technique will also straitin it self out as soon as you learn to play barre chords because eventually fast switching wont allow you to be reangleing the guitar. Just give it a try..you wil know what i mean. :)

oh yeah and practice!