I have found that "bending" the entire guitar backwards with the left hand & the right elbow draped over the body of the guitar, helps me with barres held for a long time - not sure if this approach is approved by the virtuosi here, though?!
It just FEELS as if I'm bending it, hope it doesn't snap in half one day At the end of long practice involving barres, both elbows hurt somewhat.
I also have rather short fingers; am sure they used to be longer years ago??