Do you train at home and feel there is not enough variety? To help switch things up, change what muscles you stimulate during push-ups by changing hand and foot positions. A study conducted by scientists from the University of Athens compared standard push-ups (hands shoulder width apart, legs supported on toes) with modified push-ups (?women?s push-ups,? with legs supported on knees), hands wider than shoulder width, hands together, hands above the chest, and hands below the chest. Women?s push-ups decreased the overall load by about 15%. The wider hand position stimulated the pecs more, while the triceps were targeted more with a narrow hand position. Next time you do push-ups, decide which body parts you want to stimulate more and position your hands accordingly.
(J Strength Cond Res, 19:146-151, 2005)

If you want to increase the difficulty of the movement above that of a standard push-up, use a stability ball. Start with the ball just below your knees. To make the movement more difficult move the ball towards your feet making sure to keep your body rigid and straight throughout the movement.