5-HTP...available at most health food stores is a natural supplement that is a precursor to serotonin....often very helpful in cases of mild to moderate depression. St. John's Wort is an herbal treatment that has also been found to be beneficial, as has SamE.
Make sure that you are getting sufficient B vitamins. The body requires B vitamins in order to convert the amino acid tryptophan into serotonin.
Exercise and stress management are also keys to addressing depression...stress depletes B vitamins and also taxes the adrenal glands resulting in excess cortisol in the body.
Food sources of tryptophan include meat, brown rice, dairy products, peanuts and soy protein. Turkey is particularly high in tryptophan.