As others have said, if you rule out fit, relax those shoulders and don't lock your elbows. also, don't curve your back - it is really a lean forward from the pelvis (as they have you do in Pilates). Helmets are also heavy and get used to. If you look too far forwards for too long, instead of just a bit beyond the bike, it tends to hit the upper neck.

However, strengthen the core!! Lots of times it is the lower back and core that need work. Work with a swiss ball and pilates 2 times a week completely cured my sore back.