For me a warm bath or shower, heating pad, stretching and a massage from my fiance are my go to's for back pain.
You could try taking a couple Pilates or Yoga classes for pregnant women during the week. They are very helpful to back pains. And after a few classes, you might find the position or the stretch that relieves your pain, and you may adopt it for using during your shifts as well.
I'm a Pilates instructor, and on some days I have two 5 hour shifts with a couple hours break between them (on my feet during the whole shift), and despite the break and the fact that I am very aware of my posture and have pretty strong core muscles, I sill have back pains closer to the end of the second shift... So there is no magic solution, but I know that if I don't do Pilates for a week, the back pains start much earlier during the shift.
What I'm saying is: try it. You have nothing to lose :-)
I'd try doing periodic stretches! Also, never underestimate the power of a short break sitting down if possible. If it ever gets too bad you can take a Tylenol. Unfortunately, it's a pregnancy symptoms we will be stuck with from here on out. :/