maybe knee wraps?... or widen your stance up a bit seems like it could help that....
im assuming you are refering to near max/heavy weights....
i would go with widening stance, and practice lighter, and consiously focus on knee placement and try to keep that same form as you go into your heavy sets...
i used to have this problem too and widening my stance a few inches plus knee wraps on heavy sets AND focusing with my hip flexors to drive (best described by pushing out off my heels) has helped me get better, just keep in mind, the closer you get to your true 1rm, the more your form will suffer