Zurf is all over this one, practice, practice, practice.
Very important to keep your thumb on back of the neck, so you're not just pressing with your index finger, you're pinching the neck between your thumb and finger.
On the lower frets, I tend to "roll" my index finger a bit and it makes a bit of a "C" shape"...I just seem to get a more complete barre. When I get north of the fifth fret, it seems to straighten out. Don't know if this is text book proper, but it works for me.
Did I mention you should practice? Try making an E with your middle, ring, and pinky fingers. Then just slide it down a fret, and throw your index finger across the first fret for an F. Then move the whole thing up so the barre is across the third fret (that's a G), and then move it up to the fifth fret for an A. If you want to go really crazy, move it up to the 7th position for a B!
Oh, I almost forgot....practice.
Good Luck!
Frank