You can usually tell when your mouse needs cleaned.  It feels like your mouse is moving over rough gravel.

Cleaning your mouse is easy and can save you a great deal of aggravation. Flip the mouse over to see a donut shaped disk surrounding the trackball. On most mice, a quarter turn of the disk will release the disk and the track ball.

Flip the mouse to remove the trackball. Looking inside the mouse, you can easily see the three rollers the ball rests on. Use alcohol and a lint free rag to clean the ball. Use alcohol and cotton swabs to clean the dirt built up on the rollers. (Running your fingernail across the rollers to remove heavy buildup is acceptable.) Drop the ball back into place and push down and turn the disk until it snaps back into place.

To avoid the entire problem, look into purchasing an "optical mouse."  These are becoming more popular and use light instead of a rotating ball to do their work.