What I have found is essential, is to be sure I keep my abs and my frame tight. Don't let yourself drop through your shoulders, keep your head in alignment. Otherwise, you do lose focus from the chest and it shifts to the shoulders. I can also always tell when my abs and low back are sagging, because the push-ups actually become more difficult. Think of making your body a solid board from head to toe (or knees, if your working from your knees). Move as one solid plank.