Frequent change of hand positions, good bike position, and good gloves help. Be sure to bend your elbows and not grip the bars. Fingers should be draped over the bars in a relaxed position.

Folks will often say to raise the bars, but I doubt that is usually the answer. Still you can experiment with that. Bars can be low or high and still have more or less weight on the hands. The two people that I know that had the most trouble with this on tour both had a more upright posture on the bike.

If all else fails some folks have said aero bars helped them. Also invariably the recumbent guys will pop up and say that a bent is the answer.