Key is remember to keep your shoulders above your hips always and maintain that straight back through engagement. We also have locks bind with a overhand waistband grip for a forward push NOT pull down on jersey or shorts.
My brother plays Prop and he says, as a prop you use your bulk and strength make sure to keep your head up as it has a huge effect of automatically straightening your back.
I play rugby and it's gonna be hard to get your back straight but just try not to bend your head down in scrum it'll take away some pain.