I just bought my car two weeks ago, and this happened to me twice one day on my way home one day coming home from work. Just starting off the line, and the whole car would jerk and clunk like you described. The first time it happened the bump was so severe I thought the guy behind me had hit my rear bumper (but he was too far back when I looked).

Anyway, I was determined to figure out what was causing this (because my car is still under the drivetrain warranty from the dealer, and if it was a clutch/transmission problem, I was going to find it), so I pulled into a parking lot on the way home and tried various clutch/brake/shifter combinations while getting the car in motion from a stop. After a few minutes I figured out I could reproduce the problem pretty reliably when the shifter was *almost* in first gear, but not exactly all the way. Enough that I thought it was in gear. I've been more careful to make sure I push the clutch all the way to the floor when going into first, and pushing the shifter all the way into first ever since, and I haven't had a problem again. I also regularly put the shifter into second and then into first when getting ready to move from a stop, like someone else suggested, because I've found it does make it easier to get into first.

Hope that helps. Oh, and I would also recommend against downshifting into first. No reason to.