First of all, shut down your computer properly.
Disconnect the mouse from the PS2 or serial port. The latest mouses connect through USB port. No matter which port the mouse is using, disconnect it smoothly.
Turn the mouse upside down and remove the ball-cover ring. The procedure to unlock cover is elaborated on the mouse, as the arrow icons will tell you to which side you need to twist it. Remove both ring and ball.
Clean the ball with warm water. The cover should be also be rinsed properly. The job is not over yet.
Use a cotton swab and dip it in the isopropyl alcohol, also known as rubbing alcohol. Gently remove all the dust with the help of the swab. Be patient, because this may take a few minutes.
In the meanwhile, you need to be careful. Do it very gently, because even a slight mistake could damage the mouse. The rollers inside should be cleaned accurately.
Wait for the ball and cover to dry before putting them back on. Reconnect the mouse and enjoy better performance.
If you are using a laser mouse, the more focus should be on cleaning the pad. Use damp cotton to get rid of all the dust.
Similarly, use swab to clean the entire mouse. If you find out any dust in the hole containing the laser, use a Q-tip.
If you have accidently got liquid inside the hole, let the air dry it. Once it is completely dry, you are ready to connect it to the system.