Whether your mouse is starting to wear out, or you just can't find one that seems comfortable to you, you can make your own grip by molding some Sugru. Gmjhowe shows us two different ideas for modding your mouse, both of which are pretty well done. You might have to work pretty hard at getting the Sugru smooth if you want it to look a little more polished, but you can't argue against its effectiveness. Hit the link for more detailed instructions.
Sugru Mousegrip [Instructables]