Make sure your form is good, first and foremost. Don't let your knees cave while ascending. Remember to "spread the floor" (i.e., drive with your heels and try to push the floor apart, making sure to push your knees outward, enough so they stay vertical). Work on sitting back more, as well, if you aren't used to squatting that deep. Use a foam roller to try and roll out your glutes so you can sink into the hole better.
Those are just a bunch of ideas off the top of my head. I'm sure some of the others will be able to give you a better answer.