It's been a while since I've spent this much time at my jeweler's bench, and my body is feeling it! It often feels easier, at the bench and elsewhere, to slouch and hold myself in a way that ultimately hurts. In the short term it may feel more comfortable to slouch, but by the end of a couple hours, my body is screaming.
There are some short term solutions that can help:
1. Get a chair that has adjustable height so you can always sit at the appropriate height for the work you're doing- eye level for sawing, a little higher up for sanding.
2. Get a good bench light so there's less need to crane your neck and squint.
3. Take mini breaks to stretch, especially hips, back and shoulders/neck.
4. Pay attention to how you're holding your body as you're working. Notice when you start slouching, and straighten up.
The best fix though is a long-term project: increase core strength! Slouching means your abs and back muscles aren't pulling their weight, and as a result you are resting into bones and joints, pulling at your shoulder muscles and ultimately tweeting your back, shoulders and neck.
Paying attention to how you are holding yourself will not only help reduce pain, but I see improvements in the quality of my work the more mindful I am of how I'm working. It's a means of direction attention to the task at hand, and ditching all the other trains of thoughts crashing into each other.
Quite that mind down, and focus on the task at hand.