I use cheap anti-glare, anti-fog eyeglasses cleaner I buy at the drugstore. Formulated specifically for polycarbonate, which is what CDs are made of. I use a cotton ball to apply the cleaner and a different one wipe it off. You have to do a good job of wiping---the cleaner can streak---and it washes off.
Noticed a sound improvement right away, both on my Rotel and on my Sony CD Walkman.
Jeffery