Tryptophan supplements and vitamin B6 are involved in the production of serotonin so adding them to your diet will most probably increase production of serotonin.
Eating foods rich in carbohydrates also stimulates production of serotonin, but this depends a lot on your metabolism and life style (ie how active you are).
Some people also suffer from a deficiency in the enzyme that converts tryptophan to serotonin, and that can also have an effect.