http://www.youtube.com/watch?v=cKDgacf16zo
I have trouble progressing consistently on Deadlifts because I always lose tightness in my back while pulling. I never stiff-leg the lift, but my back always handles some of the weight.
Older video:
http://www.youtube.com/watch?v=zrdLWsn-Q8k
I find this odd, because I have absolutely no trouble keeping my back tight while Squatting - and the issue is that I'm Squatting 290 lbs 3x5.
As for belts, I can't seem to figure out how to use one while Deadlifting. When I put the belt on while standing up, it fits extremely tight. When I get into the starting position for a Deadlift, it goes loose and no amount of pushing my abs seems to tighten it up. It gets so loose it slides up my stomach and back.