A moisture-wicking T-shirt keeps you dry and much cooler under your vest! The shirt lifts sweat off the skin, keeping you dry and comfortable. I wear a compression (form-fitting) Champion T-shirt, which I can get for $12.99 at Target. Get the shirt a size smaller than normal to keep it form-fitting. Moisture-wicking shirts are made by dozens of companies including, Under Armour, Nike, and Reebok. Flame resistant moisture-wicking shirts are also available for extra protection.