Use the Rollaway Technique
During their setup, a lot of people suffer from "turtleback" or rounding. Now, that could be caused by a weak lower back, which causes your hamstrings to overpower things. But the problem may stem from the way you approach the bar.
Instead of setting things up from the top down (by reaching down to the bar), set up from the bottom using a "rollaway" technique.
Squatting deep and letting the back round will allow the hamstrings to release control of the pelvis. Since they're no longer in the picture, they'll allow the low back to step in and extend the spine the adequate amount for the correct starting posture.