"Simply take a sheet of clean white paper, put it on top of the mouse, and press down on the scroll wheel with your finger, but with the paper between your finger and the scroll wheel. Press quite hard with it and at the same time scroll around – moving it up and down and round and round. Move to a different area of the paper and give it another go".

Taken from this post: http://macintoshhowto.com/hardware/how-to-fix-the-scroll-wheel-on-your-apple-mighty-mouse.html