In How To Mouse Goofy, Meg Hourihan offers an interesting story about how she learned to use the mouse with her left hand (she’s a natural righty).
The “goofy” in the title is a skateboarding reference. Most skateboarders ride with their left foot on the front of the board; this is called the “normal” stance. Riders who put their right foot in front are said to skate in a “goofy” stance. So in this case, “goofy” just means “opposite of normal”.
Switching mouse hands is something I’ve done myself. As Meg’s article says, it does take time to get used to the new setup, and it’s easy to get frustrated. But the payoff is worth it.
Once you’ve learned to mouse with either hand, it becomes possible to switch back and forth to minimize the stress on both hands. I use a wireless mouse, which makes this somewhat easier, as there’s no cord to fuss with.
Note: There’s also a nice discussion of goofy mousing in the article comments – check it out.