From your toes, from your knees, using a bench, clapping, it does not matter what kind of push up you're doing, it just matters that you are doing it and doing it correctly. The main thought with this is moving the body in an integrated way, i.e. not snaking, flopping, breaking or whatever other term you have to describe the body not moving as a unit. A way to think about this and you can ask yourself the next push up you do, when you first go to push yourself back up, do your hips move? If they don't you're not "integrated." The force generated in the push up should be immediately applied to move your center of mass or hip region. If you're leaving the hips behind (on the ground), you're most likely snaking up in a disjointed way.
There was some very nice push ups going down this morning, check out this shot of some great positioning.
3 x 6 Shoulder Press
4 minute clock:
400 meter run and 15 chest to bar buy-in
with remaining time: 3 set max effort push up.