It's the thumb folks.
The thumb is the key to playing all barres, F being the most common. You actually have to use it to your benefit - by pressing it up against the neck - help the index finger press down. Not the tip but the entire index finger as it lays across the whole fret - pressing down all six (as if it were a capo). You can then use the rest of your fingers to shape out what would be an E major had you not barred the first fret.
"You know," said Arthur, "it's at times like this, when I'm trapped in a Vogon airlock wth a man from Betelgeuse, and about to die of asphyxication in deep space that I really wish I'd listened to what my mother told me when I was young."
"Why, what did she tell you?"
"I don't know, I didn't listen."
from the Hitchhiker's Guide to the Galaxy [Douglas Adams]