I also have a trackball that is beginning to fail on scroll down. I searched and searched and there are complete tutorials on how to disassemble it and clean, but I don;t think it's necessary.
This other methods may be easier and less invasive to clean, whenever they suggest QTips, I would use the sponge ones, not cotton:
http://supportforums.blackberry.com/t5/Other-BlackBerry-Smartphones/Trackball-not-working-quick-trackball-cleaning-how-to/td-p/256520