IMO, I wouldn't do the knee version - it's really a different exercise and not going to further your development. I started doing them on the stairs, and then I'd move down one stair for one or two reps, gradually adding more reps on that lower stair, repeat until done!
Can I link to my blog here? Not sure that's okay, but I wrote a whole post on it, for what it's worth...
http://weightandpaint.blogspot.com/2...g-push-up.html