The thumb position is key, though the whole thing is about practice. I finally managed a decent Barre chord after taking on some songs that are easy, open position songs, but with one basic Barre cord - like Maggie May, which is all A, G, D, and Em except for a single tricky little F#m7 (at the point where the lyric goes "Couldn't have Tried..."). That strengthened my grip and got my fingers used to whipping around from open position to a Barre chord. Handle a couple like that, then move up to a song where there is a sliding series of Barre chords, like REM's Don't Go Back to Rockville, where the lick that starts "I know it might sound strange..." goes:
B-VII A-V G#m C#m-IV Bm A Asus4
That will get your fingers used to moving up and down the fretboard smoothly.
I'm no Barre Chord expert by any means, but that's the sequence that's gotten me to the point where I can convincingly fake my way through it most of the time.