1) Put a capo on, and practise about two or three frets above where the capo is (try the capo at the 4th fret).
The capo will make the strings sit at the 'fretted' height, rather than at the height determined by the nut.
2) Make sure your finger is straight when barring, and doesn't arch underneath the finger joints. This is common.
3) Make sure your barre is as close to the upper fretwire as is possible. Barring in the middle of the fret means you have to push harder.
4) Try to apply just as much pressure with your thumb as you do with your finger. Imagine the guitar is just a hunk of wood getting in the way between your thumb pushing against your finger. Many people push too hard with their finger, and apply no pressure with the thumb.
5) (this one will hurt!) The weakest points of your finger are under the finger joints, as this is where the fingers naturally raise - it can usually be heard when someone is barring by buzzes on the 4th and 3rd string. Feel for the bone on the underside of your finger at these joints - line the bone exactly up with a wound string (e.g. the 4th), then push it in with your other hand
This will sting a little to begin with, but what it does is teaches your finger to adapt to being completely flat.
Some people call this last one torture
haha
