There is someone on the rei website who had a similar problem with the niterider slipping out of position. He contacted NiteRider, and they gave him another bracket which fixed everything. I'll post it here:
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I bought a MiNewt 600 back in November 2011 (it's Feb'12 now), and have been using it ever since on my half hour daily commute home on unlit paved trails and lit streets. The light is plenty bright even on clear moonless nights @15mph; it's even bright enough on the 'Low' 275 lumen setting. The high setting should be good enough for unpaved trails. The light is easy to remove from the mount, so I bring it into work with me. It's easy to charge. All of this rates 5 out of 5. Unfortunately, the toothed handlebar mount that came with the light doesn't quite work; I squeezed it onto the handlebars as hard as I dared, but the beam still shifted around a bit when I hit a bump. Plenty of on-line reviews touch on this point. But wait… there's more! I sent a friendly e-mail to NiteRider, asking if they had any suggestions for how I could fix the problem. They offered to send me a different, thumbscrew tightened mount. I just got it today. Problem solved! My MiNewt now rates an honest 5/5, and an extra 10 points goes to NiteRider customer service."