My fretting hand thumb always wants to clamp against the neck harder than it has to. Sometimes it's not a problem, but when I'm holding a barre chord for awhile or doing some sort of legato sequence, it tends to press against it harder and harder until I notice the heel of my palm is getting tired. I believe this negatively affects my playing and my endurance.

I've tried to pay attention to this when I am practicing and it seems to be getting better, but I was hoping someone had more specific advice for this problem?

pretend that your guitar is alive and that by pressing on the neck too hard youre choking it
it helps it you name it
Yes this is specific. It sounds like regular tension. In our development, some kind of tension usually creeps into the body and you don't even realize it (until it hurts). I been there myself.

Really everything you do should be done relaxed, with minimal effort and a light touch. The way I get rid of unwanted tension is to slow everything way down.

At 40 BPM or so, I find that all the subconcious habits and tiny details of my techniques come under a microscope for clear examination by the concious mind --- especially tension! I discovered and weeded out some very strange muscle tendencies that I never would have caught practicing at even moderate tempo.

Repeat the problematic pattern at low tempo, and wait for the tension to appear. When it does, stop and focus on it for a moment. Release that tension and continue at the same tempo, nice and relaxed. Repeat this cycle until you can keep going with no pain or tension. Then go a little faster with the same method. It can be a slow go, but trust me it DOES work.

If you can train your brain to be on guard for tension at all times - you can always fall back on this method and un-learn any bad habit. (Both hands, both arms, both shoulders, and the neck and face)

also try bending your hand more when you play like have your thumb not hug so tightly and make just the tips of your fingers press down the frets. (Obvious i know but try it more and it seems to make sense)
Thanks for the advice guys. I backed my metronome down from 60 BPM = quarter note to 40 BPM = quarter note, well, actually 80 BPM = half note, but that's the same thing. Just helps me keep the rhythm tighter.

I found stretching the shit out of that thumb before practicing helps, as does stopping when you feel tension... in the past, I had just been trying to make the tension go away as I play with some success. Stopping everything works a lot better.

Also Carvin.. by bend your hand, do you mean my wrist? The first joint of my fingers? I don't know what you mean. I did experiment with moving my thumb from the middle of the neck up more to the side. Seems to ease the tension, plus it's easier for me to go from the classical left hand position to the blues left hand position.