Push-ups.

Wall Push-ups
One disadvantage of using knee push-ups to work up to full push-ups is that you don't get to practice holding your entire body straight. Wall push-ups allow you to build your core muscles gradually, as they must hold you steady in increasingly more difficult positions. Place both hands on a wall, slightly below chest level and slightly wider than your chest. Take a few small steps back from the wall, keeping your body straight from head to heels. Let your heels come off the floor as you tip your entire body toward the wall, bending your arms. Stop before your forehead or nose touches the wall, use your arms and chest muscles to push yourself away, and repeat.

Bar and Bench Push-ups
As you get stronger by doing wall push-ups, you'll be able to place your hands lower on the wall. This makes the exercise harder and, in turn, challenges your body to continue getting stronger. There comes a point when you can no longer shift your hands lower on the wall; they might slide off and cause you to fall. Switch to doing push-ups on a Smith machine bar instead. As you get stronger you can move the bar progressively lower, or switch to doing push-ups on the side of a weight bench instead. Bar and bench push-ups work exactly the same as wall push-ups; just put your hands on the bar or bench and walk your body back until you can lower your chest directly toward the bar or bench, then press away again.

http://www.livestrong.com/article/346429-progression-to-full-push-ups/