Push-ups are a great exercise, but beyond a certain point (20-30), you shouldn't try to make progress by doing more of them. Instead, make them harder. Parth of At Home Intense Workouts suggests six ways to make push-ups harder.
... Here are a couple of other ideas to make your pushups harder:
Stability Ball Pushups
Get into pushup position with your feet on a stability ball. Perform the pushup. This is a tough variation, especially if you have poor abdominal strength. To make the movement easier, place your shins on the ball. Read more