You do not want your ancient coins to look new. Don't try to remove the patina and get to bare shiny metal. The patina, which is a green coating on most bronze coins but may be many different colors, is part of its character. A coin with an attractive patina is often worth more than a similar coin without a patina.
Don't give up right away on dirty coins that appear to be slugs. Often there is much more detail that you expect under the dirt! We have had a few customers write that they were disappointed because they believed that they had received a lot of slugs. Invariably they hadn't used the cleaning instructions below and were looking at the dirt and thinking that it was the surface of the coin. Of course we offered a refund, but we also recommended they check this page and try cleaning them first. Every time these customers have written back to say they were wrong and that they were very pleased with the coins.
When you receive your coins, soak them in soapy water for a few hours. TSP works better than ordinary soap. TSP is a powder that you can buy in most paint and hardware stores. Mix one tablespoon of TSP with three tablespoons of warm water (increase the quantities as necessary to soak the number of coins you have). Put the mixture in a sealed container and shake well. Soak the coins in TSP solution for 5-10 minutes. Then rinse them off, dry them and see how they look. If they look nice stop. Often they need much more work.
Use a small denture brush or soft brass bristle brush and brush them gently. FORVM sells a good brass brush made just for this purpose. If you are lucky enough to get a silver coin, do not use the brass brush on it - see the instructions for silver below. Just brush gently, if the coins do not come clean easily stop and go to the next step. For the coins that do come clean easily and look nice, rinse them off and dry them - they are done. Most coins will not come clean with only this first step.
Soak your coins in olive oil. Some people soak their uncleaned coins in WD40. Few people have the patience to soak them long enough. How long - at least one week, but longer is better. After soaking, start over with the soap (or TSP) and water and the soft brass brush. If they look nice stop. If they are still dirty, soak them in olive oil or WD40 again and start over.
A dental pick is very useful for getting the dirt from corners of the design and legend, but you must be very gentle and careful not to scratch the coin. FORVM sells dental pics. Hardwood splinters, toothpicks, and Q-tips are also useful. If you use a brass brush alone, you will likely remove the patina and hit shiny metal on the high spots (a very bad thing to do) before you get the dirt out of the corners.
It can help to put the coins in the oven at 200 degrees (no hotter) for 10 minutes after soaking in olive oil and cleaning them with soap (or TSP) and water.
SILVER COINS: Silver coins can be cleaned using vinegar or lemon juice. Don't leave coins soaking in vinegar or the lemon juice for more than 5 minutes. Use a Q-tip, toothbrush and towel to clean them. Carefully wash them with soap and water to ensure all the vinegar or lemon juice is removed. To be safe, it is best to soak the coin in distilled water with a little baking soda added for two weeks, changing the water every few days, after cleaning with vinegar or lemon juice. (Never use vinegar or lemon juice to clean bronze coins.)
Silver coins can also be cleaned by placing them in an aluminum can filled with a baking soda and water mix.
STORAGE: Coins should be stored in archival safe materials. Most plastic and paper will deteriorate in time and damage your coins. Even cheap coin flips contain PVC that will severely damage coins after long term storage. FORVM carries PVC free archival safe flips. These flips are about twice the price of inexpensive plastic flips that contain PVC, but they are safe for long term storage of your coins. FORVM's flips have two pockets, one to hold your coin and the other to hold a paper insert on which you can write or type information about the coin.
http://www.dirtyoldcoins.com/restore/cleaning.html