You may want to look into arched back good mornings as a tool to teach you to maintain back arch.

That being said, flattening of the back isn't bad. What you don't want is rounding, as typically once you are rounded you lose tension and that is when bad things happen. So long as your lower back is staying tight, then a flat back isn't a bad thing. I can tell you that I am not fllexible enough to maintain an arched back through a full squat, but I can keep a flat back.

Anyway, back to the arched back good mornings. Set the pins in your power rack to just above belly button height. Unrack the weight (light to start with) and while keeping an arched back, bend forward at the waist. Some knee bend is fine, but the bar should end up in front of your knees. How far in front is not of much relevance. In a squat the bar stays over the heels, but this is not what you want. You want to create that imbalance. You want your lower back and hips to do the primary moving, not your thighs. Anyway, lean forward, arched back until you hit the pins, then stand back up. Simple.

I find that lighter weights (something in the neighborhood of about 50% of my max squat) for high reps works best for me. There are lots of guys that can good morning weights approaching 90% of their max back squat... but I don't think that this is necessary as you are just looking to fix an issue with the GM's, not set a PR in GM's.