The basic idea of barre chords is that your hand becomes a capo. The key issue is developing that muscle right at the base of the thumb going into the palm of your hand. Funny enough, that is almost the exact place where a capo forms its tension.
A specific question I got was how to hold the A flat Major barre chord. I've seen methods where people use all three fingers, as if holding an A Major Primary chord plus the first finger to hold the first fret. I don't opt for this method because using the ring finger allows me to play suspended chord adjustments with my pinky, while the ring finger lets the octave note ring out. This also is a fast and easy way to play the chord based off of standard power chord or Root 5 forms.
The key to using the ring finger and allowing the F note on the E string(first fret, first string) to play cleanly is to bend the pad of your ring finger. Once you develop that thumb strength, "barre-ing" that entire first fret will get easier and easier.
I don't know if this is the best way to do it, and it's definitely not the only way, but it works for me.