I have an Idea: Switch to drop bars and bar-end shifters. Keep your hands on the hoods and don't move them around much. This can help. Keep your hands relaxed, and try to rotate them in circles. Again, keep the hands relaxed, and DON'T CLICK THE BRAKES. If you don't HAVE bar-ends, then look for old-style brake hoods for the levers. These were designed for bar-end shifters and have a larger gripping surface. And, of course, get a better fitting bike.