Well to begin with, just make sure that your legs absorb some of the impact before you even begin to roll; A lot of people always just try to roll as soon as their toes come into contact with the ground! So when you absorb impact with your legs, try not to go beyond 90 degrees, as soon as you hit around that 90 degree mark, you should begin rolling! Another hint is to be leaning slight forward when you come to the ground, and always land on the balls of your feet!