Practical tape head demagnetizers are available for under $10. Try to find one with a plastic coated tip. If you can't find one which is plastic coated. you can slip a drinking straw or plastic tube over the tip for the same effect. This plastic will prevent the demagnetizer from scratching the head.
Before plugging in the demagnetizer, remove all tapes from your working area and unplug the recorder. Hold the demagnetizer away from the recorder as you plug it in. Slowly bring the tip of the demagnetizer up to the tape head and slide it back and forth across each tape head for five one-second strokes. Then pull it away from the head slowly and go on to the next. After demagnetizing the heads, use the tip on each metal tape guide with a similar five strokes. Last, slowly pull the demagnetizer far away from the recorder and unplug it. Recording engineers use a demagnetizer before each recording session.
(courtesy of AudioFAQ)