I had a problem with this and never realized it until my girlfriend brought it to my attention. Eventually, I experienced some jaw discomfort and, oddly enough, my teeth weren't lining up correctly with my jaw closed (this was surprisingly distracting).
I tried getting a night time mouth-piece to prevent grinding teeth. It didn't help and I would recommend against trying one. After sleeping with it in, I found that I couldn't close my jaw the next morning (which was REALLY distracting).
This might sound corny, but I finally stopped grinding my teeth by making a mental note to myself before falling asleep every night, telling myself that I wouldn't grind my teeth. The power of self-suggestion seemed to solve my problems.