This is the barre system that I have taught to many students:

1. While learning, adopt a classical or semi classical position with the neck well above the horizontal, and the face of the guitar vertical.

2. Get in close to the guitar with your elbow underneath the neck.

- these two points minimise the amount of bend in the wrist reducing the chance of carpal tunnel problems -

3. Don't rest the guitar neck on the palm of your left hand or the base of the first finger - there should be a little "tunnel" between the neck and the V of the finger and thumb.

4. While learning a shape, place your 2, 3, 4 fingers before you put down your barre - then straighten your finger and lay it across the neck without pressing down at all - there is no point tiring your fingers holding a barre before you play the chord - then press and play.

5. Les.B's right - the strings are easier from frets 3 - 7 so practise the shapes around fret 5.

6. Muscle development does help, but the best development is in playing barres; concentrate on the muscle between the thumb and forefinger (this is underused in most people).

7. Play (say) a twelve bar as practice with open E, A fret 5 and B fret 7 to give a brief rest while building the skill.