Note to readers: The methods I have placed on this page work for me and are the best way I find to clean the objects in question. I will not take blame for any mistakes you make when cleaning your finds in the ways listed below.

Bronze, Copper, Brass, Lead and Pewter:
Wash and clean in warm soapy water, use an old stiff toothbrush to get as much dirt off as possible. If really stubborn dirt cannot be removed after brushing, leave the items to soak in a plastic container filled with household paraffin. Remove the item from the paraffin from time to time and brush with the toothbrush to get into small crevices and lettering in coin legends. However, be very careful not to scratch too hard and end up marking the metal or any patina that is present.

Sealing and Preserving:
To preserve copper or bronze with a nice patina, give a good coat of Patina Cream - use either brown or green. Brown is a little difficult to get hold of, you can instead give a good coating of liquid paraffin, let it dry and then give a further coating.
If bronze or brass items are pitted and with areas that are crumbly, these areas need picking out first and stabilising with a coating of benzotriazole - leave them to dry and then give them a final coating of Intralac. A good way I find to preserve coins and bronze artefacts for a short period is to give them a coating of Vaseline. This helps to protect the surface from moisture and prevents further decaying of the object.
Lead and pewter items should be soaked in household paraffin and lightly brushed from time to time. After drying, coat with liquid paraffin, let it dry and then give a second coat.

Silver:
As with gold, wash and clean in warm soapy water and dry off. Further cleaning may not be necessary, however, most detecting finds will. A good way I find to clean hammered silver coins is to slightly wet them and hold them between two small pieces of aluminium foil. If you try this method, don't worry if you smell rotten eggs because this is hydrogen sulphide gas being given off as the object cleans. Remove the coin and gently rub scouring cream on to the coin. The result if done correctly should be a coin with toned background and shiny raised surfaces. Another way to clean silver means you need a small plastic container. Cover the bottom of the container with kitchen foil and put in half a teaspoon of washing soda over this. Place the artefact or coin on top of the kitchen foil and cover with boiling water. Remove the item and clean again in warm soapy water with a soft toothbrush.

Gold
From my experiences, Gold coins and artefacts usually come out of the ground as clean as the day they went in. All that I have found necessary is a wash in warm soapy water with a soft toothbrush. If there are any small stubborn pieces of dirt stuck to the object, these can usually be removed with a toothpick. In general, old gold usually develops an attractive slightly matt orange tone, which in my opinion should not be removed.