Inside the mouse you should see three rollers. Two of these rollers translate the mouse movement into instructions for the computer so the cursor can move around the screen. The third roller helps provide balance to the ball within the mouse.
These rollers can get extremely dirty thanks to all the dust and grime they pick up from the mouse ball while rolling for endless hours over your mouse pad. On that note - cleaning your mouse pad regularly can do wonders for keeping your mouse clean.
Using a tissue or cloth with some cleaning liquid on it, clean the rollers until all of the debris is removed. A fingernail works well also, without the cleaning liquid of course! When you're sure that every bit is gone, replace the cleaned mouse ball and replace the mouse ball cover.