When pushing in a circle or even before a cross its very important to stay low so you have a long, extended push.
To ensure that you get the best push, you should try for a 90 degree knee bend, but thats only half the equation.
For a strong push you must push through the center of your frames using your heel and the balls of your feet.
Put power into your pushes by extending your leg and using the muscles in your hips, quads/hamstrings, and calves.
While pushing make sure you are pushing with all your wheels, and when finishing your push all your wheels should leave the floor at the same time.