When I have this problem I put a very small quantity of surgical spirit on a soft cloth and rub the ball with the cloth whilst holding the mouse upside down. This will remove the grease from one's finger from the ball and has always solved the problem.
I had this problem a lot so stripped the mouse down, any alcohol technique does not get to the innermost bits, this is not as complex as it sounds so long as your eyes are good 8-0 See here http://web.mac.com/karelgil/MacWebSite/MightyMEng.html
Ended up doing it so often to remove the fluff I ditched it a year ago and now use a Logitech wifi mouse.