i'm not too strong, but deadlift form is something i pride myself on and others often ask me to help them with their deadlift. The biggest mistake i see is people rounding their back at the end of the lift; your body should lockout simultaneously- legs, hips, back all at the same time. Make sure you squat enough, and steve brought up a good point to keep your arms straight. if you can video yourself, we could probably help you pinpoint exactly what's wrong.
Also, i'm going to respectfully disagree with steve to watch people DL at your gym. Sure, some people definitely have good form, but 99% of people in the gym have shitty deadlift form. I saw 3 people DL today at my gym and every single one of them rounded the shit out of their back. Just be weary of the advice you get at the gym because a lot of it is bullshit