If your soup is too salty, first try adding an extra cup of water or broth to help to thin out the soup. You can also add potatoes or a cup of rice or pasta to absorb some of the liquid, including the salt.
Starches such as potatoes, rice and pasta work to absorb liquid and thereby thin out the salt content in the soup. To avoid making a soup that ends up too salty, resist the urge to add salt before the soup is done simmering. When you are about to serve the soup, taste it and add salt as needed.