Tips for preventing back pain when working on your feet?

So I’ve been at this job for almost 3 months now, but since I’m in college I’m only doing part time 4-5 hour shifts. Anyone have any tips to avoiding back pain? I have sciatic never pain that comes and goes, and I notice when I’m standing for 5 hours I start to feel tense and almost like spasms where it hurts to breathe in or bend over or even pick something up