I second those suggestions. With push-ups, it's all about practice at first, much like how it is with pull-ups. It sometimes takes a while to get things going to where you get the strength to start doing them, but just work with the assisted ones on the knees and with doing negatives (get to the part where you're set with arms fully extended any way you can, then work on lowering slowly for time, trying to do more negatives or slow the descent until you build the strength for full ROM push-ups), and things will get there in time. As you start to get a bit stronger, you can also do decline press style push-ups (it's like doing a decline bench press, but opposite in that you'll have your torso higher than your lower body), using a stairway to elevate your torso about 20-30 degrees (usually 2-4 steps up is where you'll position your hands) which will make the movement a bit easier and help transition to doing them flat on the floor, too. I always feel like it takes off about 20% of my weight when I did them like that, definitely could do them easier than flat on the floor. Last option would be to rig up something with rubber tubing for some assistance with push-ups, similar to this device that's used to help with bench pressing assistance:http://youtu.be/fWojAlnbudM
(still can't get videos to embed properly, otherwise I'd take get it posted here properly!)
The Slingshot is a great training tool (love the one I have), and I've heard of people using rubber tubing to rig up similar things that will allow a little rebound off the bottom part of the ROM to help get some assistance on push-ups. There's a lot of ways to get that strength built and get your body used to the movement!
It's like when I first started trying to learn how to do pull-ups correctly, I could barely muster two even with a resistance band under my knees to take off about 50 lbs. at the start of the lift. But, after hammering out singles and negatives for about 4 weeks, it took me to being able to do sets of 5 reps in a short time. Sometimes it's just a matter of building a bit more strength and getting your body used to the movement, then one day it can surprise the heck out of you by having once-difficult lifts feel much easier!