I notice that no one here has mentioned the difference between "OCS pushups" and fleet pushups. The fleet regs are generally pretty lax, but if anyone that reads this is training for OCS or any other program which will be on the strict side, they add additional requirements which I haven't heard of many other commands using.
At OCS, we had to keep our fingers grounded together (no gaps), pointed straight forward. Back straight, head and eyes facing forward (parallel to the ground, you shouldn't see the grass). They were VERY STRICT about this. This next part AFAIK is universal, but is much more strictly enforced at OCS: go down so your tricep is parallel the ground, and go up so your elbows lock out. When you get tired you really have to force yourself to complete the full rep, and when a DI is standing over you, they won't hesitate to knock 20 off your count for 3 or 4 crappy reps. People have failed the PFA because they didn't pay attention to form.