The built in degaussers in TVs and computer monitors work fine but are only a fraction as strong as a TV man's outboard degausser.

Bringing speakers too close that are not shielded, or the wife running the vacuum cleaner too close can leave magnetism that will distort the path of the electron beam(s) and cause 'splotches'.

Once in awhile the tube will have a defect where taping small magnets to the back of the tube are needed to get the remaining bad spots out. Manufacturers will touch up the set this way occasionally when they are about to leave the factory.

BTW, the best way to see whether you have a problem with this (called 'color purity') is to set your DVD or receiver's onscreen menu background to RED. The red gun shows purity problems better than the green or blue. Trying to determine if the purity is good with moving video is pretty tough.

Larry's comment about turning it off is good. Just like in the days of demagnetizing tape heads on the cassette or reel to reel: Turn it on away from the unit, bring it slowly in, then slowly out with a circular motion. Only when you are a good distance away should you shut the thing off. You can have the set running when you do this - the colors reproduced can keep the kids out of trouble for a couple minutes!