There are a variety of ways to make pushups harder, from doing them at a decline to trying other advanced options such as divebomber pushups. Another option is to add a jumping jack. Yes, that's what I said.
Actually, this move is more like doing a jumping jack on the floor, allowing you to combine cardio, power and core strength all in one amazingly challenging exercise.
Clearly, this exercise isn't for everyone and it's very easy to strain your back if you're not ready for it. You should work your way up to it by keeping the movement small and/or bending the knees or, if you looked at this exercise and thought, "Really? This is your exercise of the week? You should be fired," you should probably skip it altogether.
Do it right: In a pushup position, with hands slightly wider than shoulder-width apart, jump the feet apart as you bend the elbows into a pushup, going as low as you can. As you push back up, jump the feet back together. Continue for 30-60 seconds or until your body gives out and you collapse to the floor.