There are three things you can do to alleviate this annoying problem. First, you may be gripping your bars too tightly. Your grip should be very light, basically just enough to provide upper body stability unless the road is rough. Sometimes I don't even wrap my fingers all the way around the bar or brake levers, but just sort of rest the "V" between my thumb and first finger on the hoods. Second, you can try gel gloves. Personally, I like the Pearl Izumi Gel-Lite gloves. They provide a bit of cushion without losing much feel. Third (and this is the only expensive solution), you can get carbon bars. Carbon offers some vibration damping effect as well as being lightweight. But, most of them are in the $180-250 price range. Good luck.