Your shoulder blades need to hit the deck on each repetition. Your butt cannot come off the deck, and your arms cannot come off your chest. Your heels can be as close to your ass as you like, and you need to touch both elbows to your thighs on each rep. MANY people cheat on this event in my experience. Do them correctly and when it comes PFT time, you'll be good-to-go. Do them everyday like O-man said. I do them every other day now to maintain. I do 100, wait 2 minutes, do 80, wait 2 minutes, 60, then 40 then 20. It's tough but if you can do that, it'll keep you PFT ready. Works for me anyways. Same goes for pull-ups-- do them correctly, then worry about your numbers. I've seen candidates at OCS rip out 12-15 at the initial PFT, and the instructor did not like any of them. Sucks to be that guy. Hope this helped. Semper Fi.
Lt. Williamson