If you have or have had rotator cuff injuries, you might be getting the nerve-pinch that bothers me so much. I do 3 things to help. While riding, I remember from time-to-time to sit up, let loose one hand, and shake that arm from finger tips to shoulder a few times. Helps restore circulation and feeling. At every stoplight, and at stop signs if I'm help up, I'll stop and roll both shoulders a few times forward and back, and shake both arms from finger tips to shoulders as much as possible.
Last thing I'm doing, which also seems to help, is some light weight training to strengthen shoulders, back, and torso. Core work seems quite important for a bike rider, strange as that might sound since we're usually sitting. I've found the more I work on core (abdominals and lower back), the better I can ride and the fewer issues I have with neck and shoulders (including hand numbness) while riding.