Another thing to try is vitamin B5 - pantothenic acid:

"In addition, Pantothenic Acid acts as an adrenal gland stimulant. As the glands are stimulated, the production of adrenal hormones, and especially cortisone, increases. The vitamin is also believed to be capable of reducing fatigue and thereby enhancing stamina levels. Pantothenic Acid is crucial for the production of antibodies too. Many specialists agree that Vitamin B5 can help the body fight off the symptoms of stress, anxiety and even depression."