Use a [plyo box][1] or something similar. So you put your hands on the top edges of the box and do a push up as normal. Use a shorter box to make it more difficult until you can do them off the ground. I feel this is better than knee push ups if you need to scale the movement since to me they feel closer to real pushups.


  [1]: http://www.kettlebells.com.au/store/plyometric-boxes.html