i was watching FitTV (we get it at my college for some reason) and they talked a/b common mistakes with pull ups. most people try to pull themselves up with their arms which is incorrect. they said you need to lean your body back a bit and use your lats in unison with your arms. they showed guys who just used their arms and did like 7 pull ups, and then once they got proper form they did like 18. once again just goes to show that good form always prevails
i tried it in my gym the other day and i did a few more than usual. hope this helps