[Jim Owen]

Hey Rockguy,
Here's what I did years ago--it was the old timey way. Practice the full barre with the force of your other three fingers backing up your index. Make sure that each note frets cleanly.
You'll get a sense of where your thumb has to be for an effective clamp.

Devote some of your practice time each day to this. When it feels right, then practice the full barre with just index, middle, and ring.

Then, just index and middle.

Then, just index.

It worked for me.