I'm guessing we all work in some form of office - find a first aid kit, and grab one of the rubbing alcohol swabs (an isopropyl alcohol wipe)... squeeze out a little of the alcohol, lock your keyboard (or turn off the phone) and firmly rub the ball in all directions for as long as the swab will stay wet... take a lint-free cloth and rub the ball gently to take off the excess alcohol/grime, and leave it to dry for a few minutes... worked like a charm for me - beats a cotton swab any day.

Just my 2 cents...