The best solution I have found is this...
Take a clean sheet of paper, and set it down on a flat smooth surface... like a kitchen table.
Then turn your mouse upside down, and firmly roll the hell out of the little grey ball applying pressure.
Go back and forth, up and down, circles... vigorously.
You will actually see black particles coming out either on the paper or on the surface of the mouse itself.
Keep rolling the hell out if it until it works.... and it will work.