A good variation which I prefer (since it stresses my injured wrists less for me) is the dive bomber pushup:
http://www.youtube.com/watch?v=YidzZW74T2M
A great way to progress to higher #'s is the 'ladder' method:
Do 1 pushup, rest for the length of time it took to do 1 pushup.
Do 2 pushups, rest for the length of time it took to do 2 pushups.
etc
up to say, 4, or 12, or however many you want to do. You can then do the ladder back down. The idea is you're building volume without overly fatiguing the muscles, which is the whole point of endurance training, right?
If you can't do 1 pushup, start by doing negatives, where you just do as much of the 'down' portion of the pushup as you can sloooowly. Repeat until you can go all the way down, then try going back up. Keep doing the negatives, you'll build up the strength eventually. This works awesome for pullups, dips, etc.