I had this problem before mine stopped working altogether. The mac forums suggest wetting a paper towel (and squeezing it out, of course--not too wet), holding the mouse upside-down on it, and *vigorously* rubbing it around. It works.

Or rather, it worked, and then it stopped working months later. But it is a temporary fix, and just using water isn't damaging.

Get a real computer?

*delurk*

You can use rubbing alcohol instead of water – I've done this many times. It's a nice mouse except for the regular wheel-cleaning troubles. If it doesn't work the first time, you may have to repeat the cleaning process.