Whats an effective way to clean a coin?

The main rule in coin collecting is you don't clean coins. The coin will lose value. That rule is usually broken when it comes to ancient coins dug up from the ground. They usually need to be cleaned and are. To get rid of grease and grime on modern coins, acetone can be used, then distilled water and let air dry. If you don't care about the coin use a buffing wheel. One thing to remember is when a coin is cleaned it sticks out and all collectors know it has been clean, especially if harshly cleaned.

Go to a coin shop and buy some of their coin cleaning solution that they sell.

If you clean the coin, you devalue it, sometimes a lot. I would take it to a local coin dealer & get their advice.

I found that with pennys you can use a pencil eraser. The rubber eraser takes off most dirt and grime and then rubbing it with a generic metal cleaner you might find in a store should make it shiny.

We used to clean pennies with any strong sauce - HP or Daddies - I suppose Soy Sauce would do the same?

use a slice of lemon or tamarind and scrub the coin gently