One of the best things to do for just about all soils is to add things that keep it in good shape. These things are called soil conditioners and include manure, compost, peat moss, and leaves. All of the soil conditioners are put on top of the soil (about 2-3 inches deep) and then mixed into the soil.
If you can get it, animal manure from animals like cows, horses and sheep are great soil conditioners. It's best to use manure that is more than six months old. New or fresh manure might hurt your plants because it is too fresh or has not aged. Also, if you have a pet a home, don't use that manure for the garden! There are things in it that are not safe especially around vegetables that you pick and eat.