I second the proper fit advice. I had to cut my flat bars down about 1-1/2 inches because they were too wide. I also put bar ends on them, which provides more hand position options.

Make sure that you are bending your elbows. If not--you might be reaching too far (your bike does not fit you), and you are likely putting alot of weight on your hands and wrists. Bending elbows also helps with shock absorption.