Let the ball fall out. Take the ball and clean it up with some cool soapy
Inside the bottom of the mouse you'll notice some rollers. Usually there
are two thin black rollers that are long and then a larger, shorter, white
roller that has some movement to it. Were those rollers meet the ball they
tend to compact up with dust and paper bits so that they end up having
quite a compact sticky mess.
If you take your finger nail and scrape down the roller you can usually
easily scrape off the crud. With the correct motion you can make the roller
move slightly and rotate a bit. Just keep repeating that till the roller
rolls all the way around and you pick off all the crust. It may take a
little while to get off the most stubborn bits, but they all can come off.
With that crust scraped off the mouse will run like new. :)