Guitar: Barre Chord Trouble?

I'm learning barre chords now at my guitar lesson. Wow... I was told to practice the F and b flat barre chords. I have trouble though. It hurts first of all. But also, my fingers won't stay straight, they bend backwards like I'm double jointed in my fingers. Also, my fingers are thin. So, are there any tips on hand placement, finger strengthening exercises, or anything else?