Why is my mouse jumpy?
Answer: If the cursor on your screen isn't following the movement of your hand, there's a good chance you've got a dirty mouse. No matter how hard you try, it is basically impossible to keep your mouse pad dust and dirt free. And after hours and hours of rolling on a dirty surface, your mouse ball is bound to pick up some gunk. This grime gets twisted around the rollers inside your mouse and prevents them from rolling smoothly against the mouse ball. Since these rollers are what tell your computer what the mouse is doing, it's not good when they don't roll.
So if your cursor isn't moving how you want it to, before you decide to throw your mouse against the wall, check to see how dirty it is. To check the mouse, twist off the cover on the bottom that holds the ball in. Then take out the ball and examine the rollers. If you can see stuff on them, it's time to clean 'em. To clean the inside of your mouse, you can use a Q-tip or even your fingernails to get the dirt off the rollers. Make sure you clean all the way around them. Then blow inside the mouse to make sure everything gets out.
Finally, wash off the mouse ball with soap and water and dry it. It is also a good idea to clean off your mouse pad. Once you've done all this, you can put the clean ball back in your clean mouse and experience smooth rolling freedom once again.
Of course, if you have an optical mouse, this should never be a problem. =)