First, clean the mouse and trackball as according to Apple's instructions.

http://support.apple.com/kb/ht3226#peripherals

Next, take a piece of bright white paper, turn the mouse upside down on it, firmly press down on the ball, and vigorously run the mouse quickly up and down, left and right, AND in circles over the paper. After a while (it took me about 1-2 minutes), the lint and dirt that's been building up will break free of the sensors and when you take the mouse off the paper, you'll see everything that you cleaned on the paper -- and the scroll ball will work just like new.

I finished by cleaning it again with a damp, lent-free cloth.