Old coins, simply by virtue of having been in circulation for many years, are often caked with dirt and grime. Most coin collectors would rather look at a clean coin as opposed to a dirty one. While old coins can be safely cleaned, you must be very careful not to damage the coin in any way while you're doing it. On the other hand, if you intend to sell the coin, you should not clean it at all since dealers usually prefer buying coins in their original state (grime and all).
Wash your old coin softly with a washcloth, ear swab or a toothbrush (with soft bristles) and whatever cleaning agent you are using.
Rinse the coin with distilled water. Tap water may include minerals that can damage an older coin. Rinse all of the cleaning agent off of the coin.
Dry the coin with a soft, clean cloth. Pat the coin instead of rubbing to avoid scratching it. Let the coin air-dry until it is completely dry.
Gold and nickel coins should be cleaned with warm soapy distilled water, while silver coins should be cleaned with lemon juice, vinegar or rubbing alcohol. Copper coins can be cleaned safely with olive oil. Vaseline can also be used to clean coins. Do not use commercial metal cleaners. They will harm the toning that adds to the value of an old copper or silver coin. If your coin will not come clean after washing, soak the coin in the cleaning agent but remove it as soon as it is clean. Professional coin dealers often offer cleaning services. If you think your old coin is even remotely valuable, you should take it to a dealer to be cleaned.
Gold is a very soft metal. Use extra precaution when cleaning a gold coin with a tooth brush.