Hi, just wanted to add my 2 cents in also since I had the same problem in the past. I think it's definately better to do fewer push-up using proper form than to do more in poor form, plus it's not good for the body. I found that doing the bent knee position definately helps you keep your back straight. Remember that all of the movement is in the arms, just bend the elbows to lower up and down. The rest of the body should be static-nothing else should be moving. Too often, I see people doing push ups the wrong way, they are moving there butt or back up and down and not their arms. Another good way to practice good form manly style is to cross the ankles or put your feet on a raised platform like the step aerobics step with no risers-again, this helps you keep your back straight.
Eva