Use a strap, sitting or standing. It will help your playing and, IMO, quite likely help your back.

When I got tired, stiff or sore when playing I refocused on posture and [u]proper[/u] sitting in the chair. And exercise.

At the beginning and inbetween different parts of the practice session I would do stretching or other exercise (trunk bends, toe touches, leg lifts, crunches, stuff for the neck... even push-ups.). Just a few of one or the other inbetween each set. It solved virtually all of the stiffness/soreness I was experiencing and it was a refreshing break during the practice.

Guitarists have finger stretches and other warm-up techniques... why wouldn't we work on the other parts of the body that work during guitar playing?

I played better and looked better, too.