The push up is the Godfather of bodyweight exercises. It works everything form the hips up, and you can perform it anywhere. The first video demonstrates a traditional, USMC-style push up. If this is too difficult, try the progressive phases offered in the second video. But never do push ups from your knees, or "girl style" push ups. They aren't bad, they are just a waste of your time. Finally, try Zuzana's One-Leg Push Ups. Or just watch the video on 'repeat.'
Coach Stevo Top Tips
Traditional, USMC-Style
Progression
Bodyrock!