Also, it's not all in the arm strength, but rather in the back. Try your barre with your back straight (not bending over the guitar).
Notwithstanding Kevin's advice about using the thumb to press the 6th string (although I have heard you should never do this), don't forget that the thumb at the back of the neck helps alot. Use it to apply pressure to the back.
Kevin, I'm not criticizing your suggestion, just wonder why we hear two different ideas about using the thumb?
Terry