I am also trying to do the infamous barre chord, I bu tam not there yet. First thing I started doing is to barre 3, 4, and 5 strings (not including the low E) and I can do that. All 6 strings are still a problem, so I did some reading on the web last night. Several people suggested placing your thumb on the lower part of the neck. Imagine a horizontal line down the center of the neck, your thumb should be below that line. Also they suggested adding a little more force between your thumb and the neck by using your chest to push. He emphasized that this does not need to be much force, just a little. I have not had a chance to try these yet, but am hopeful.
Cordoba C 10 Cedar