

Try this:  flip your mouse over, and open up the cover that holds the
mouse ball in place.  Remove the ball, and inside you should see
(probably) 3 rollers.  If any of the rollers have a tiny layer of
caked-on dirt, dust, or otherwise unidentifiable scum on them, carefully
scrape it off with a small knife (Xacto works great).  *Be Careful*
You do not want to gauge the rollers, just clean off the dirt.
Put the ball back in, put the cover on, and there you are.  

I clean a couple of dozen of these every month here.  Another symptom
is that when you move the mouse, it seems to "click" along.  If this
doesn't solve your problem, at least you have a clean mouse.  I highly
recommend using a proper _soft_ mouse pad, and if you are a clean freak,
place it in your drawer every night or when you don't use it to keep
the dust off.

Personally I prefer track balls.  Oh, which brings me to another point:
if your mouse or trackball tracks optically, and the sunlight is 
streaming through the window onto your mouse/trackball, you may notice
that it will stop working.  If this happens to you, close the curtains
or blinds, or simply shade your pointing device, and see if that helps.
