I've noticed that when I play barre chords a lot my thumb gets really tired and sore. Am I applying too much pressure? Does it get physically stronger through practice? Any suggestions?
You could be pushing too hard, but not necessarily, you only need to apply enough pressure to hold down the strings without, too much and you'll actually make all the notes ring sharp. Stamina is built with practice, so you may just need to play more.
Where's Waldo?
Practice, but Don't push yourself or you may end up with long term problems, like RSI, Arthritis etc...
i had a similar problem that was relieved by correcting my thumb position to behind the middle of my four fingers on the back of the neck and straightening my wrist more. It took some getting used to though. hope that helps