Craig Ballantyne of Turbulence Training (www.turbulencetraining.com) says that one way to increase your strength (and therefore the number of repetitions of pushups you can do) is to focus on "… the eccentric portion or the lowering phase of a pushup."
Also, you could try to "… hold the position …" mid-way through (5 seconds), or at the bottom of (3 seconds) the movement.