It's easy to use good form on push-ups... until you start to fatigue. Instinctively, we try to make it easy to keep going by sagging the hips. This effectively lightens the load and shortens the range of motion. But of course that's incredibly bad form too.
To fix it fast, simply bring one knee up to touch the same-side elbow. That puts you back into correct alignment. Now, keep that form and continue to do reps.
If you're unsure about your starting hip position, just use this same cue before beginning a set.
Dr. Ken Kinakin, much like Dr. Mike Leahy, knows your pain. In fact, he's made a living off of it. The doc is one of the best in the world at devising training protocols to help make you injury-proof. Yeah, he likes bodybuilding and powerlifting routines, too, but when it comes to figuring out how to fix what ails ya', he's one of the best.
A body that's out of balance is an injury waiting to happen. Here are some diagnostic tests (and remedies) that even the guy who hates diagnostic tests will love.