Dip a cotton swab in some Anhydrous isopropyl alcohol and gently wipe the heads--the metal fixture that reads the tapes. Dry the heads with a clean, dry swab after.
Press "Play" on the cassette player and gently wipe the capstan--the metal spindle to the right of the tape head--up and down the shaft.
Rotate the pinch roller--the black rubber roller that guides the tape--with a rubbing alcohol-dampened cotton swab. Wipe the stationary plastic and metal guide posts.
Wipe all the parts you have cleaned with a dry, clean cotton swab.
Plug in and switch on the demagnetizer--which can be bought in an electronic store or over the Interent for under $10--away from any audio equipment. Slowly move the head of the demagnetizer towards the tape heads. Slowly move the demagnetizer back and forth over the tape heads, but don't let it touch them.
Pull the demagnetizer slowly away from the tape heads and switch off and unplug.