Slipping with flaps extended on the 172 and most high wing Cessnas can bend the flap rails. The best way to loose hight is to slow it down, select full flap then put the nose down while monitoring the flap limit speed. It comes down very nicely and quickly. Sometime a bit of 'faith' is required as you bring the nose up to loose the speed and the runway dissapears from view, but as soon as the flaps are down, push the nose over and you'll be fine.