Using a cotton swab and rubbing alcohol, gently wipe the tape heads - the square metal components. Press "Play" and clean the rotating capstan with the damp swab. Rotate the pinch roller manually and wipe with the dampened swab. Wipe the stationary metal and plastic guide posts with the cotton swab. Dry all the parts with a clean cotton swab.
You can buy a demagnetizer from an electronic store or over the Internet for under $10 as of January 2011. Plug in a switch on the demagnetizer, away from any audio equipment. Slowly move the demagnetizer towards the tape heads and slowly move it back and forth over the tape heads, avoiding any contact. Pull the demagnetizer away from the tape heads slowly and switch off and unplug.
Unscrew the screws on the front panel of your cassette deck. Remove the deck from the panel. Lift the pinch roller assembly from the shaft and replace with a new pinch roller, available over the Internet and at your local electronic, or audio, store.