Well, the best way to increase the amount of pushups you can do is...
do more pushups! Seriously though, you could do any number of chest exercises to help you in this area. Bench presses are the obvious choice. Try doing flat/ incline/ decline bench presses for a while and that should help immensely. You may want to use just a moderate amount of weight in this case since you are going more for endurance and not mass. Try using a weight that you can press 12 times or so. Hope this helps!
