Underneath your jacket, wear clothing made of sweat-wicking fabrics designed to draw moisture away from the skin and enhance the rate of evaporation. These garments are available as separate long-sleeved tops and bottoms or one-piece outfits that resemble a surfer's wetsuit.
While a number of sports apparel companies design multipurpose sweat-wicking garments for athletes like runners, cyclists and other energetic types, base layers for motorcyclists are designed to accommodate the rider's posture. For example, the tops typically feature elongated backs and sleeves to fit a rider's bent-over frame, and they frequently include perforation panels to increase airflow.
If you're involved in racing, base layers make it easier to slide into your race suit.