A user on Lifehacker has recently posted a video on how to clean the little track ball on a blackberry. (I would say it works for any mobile device with a track ball).
I tested it out on my Blackberry which was having issues scrolling to the right and it fixed it up.
All that required is a blank piece of white paper and you just need to rub the trackball on the paper to clean it. Check out the video
[YouTube via Lifehacker]
Another method that I used just after Christmas involved pulling out the track ball assembly and cleaning the rollers.