My Mouse has a Dirty Bottom

What is that stuff that keeps building up on my mouse's bottom? No matter how many times I clean it off and scrub the surrounding desk area it keeps coming back. I don't use a mouse mat because the Health & Safety Police told me it's better to use the wide area of the desk to help avoid RSI (Repetitive Strain Injury). Wise advice, but the price I pay is having a constantly mucky mouse underside.

Someone once told me that wiping a dirty bottom on a sheet of paper is a pretty good way to clean the gunk off. It's true, it works. Any other advice from fellow mouse masters or should I just resign myself to giving it a good polish every day?