Squat Univerity on improving push ups

SquatUniversity on correct push up technique and How To Get Better At Push-Ups In Weeks

 

Beach push-up (or get up)

Original Strength's The Beach Push-up for Age-proofing Your Body

Squat Univeristy on the Perfect Push-Up

 Squat University on The Secret to PERFECT Push-Ups!

How not to do push ups

 Tykato Fitness, who I believe is a physio, has a video STOP DOING PUSH UPS WRONG! (You Are Destroying Your Shoulder!).

And Antranik shows you how to do them correctly: How To Do Push Ups with Antranik.