As always, there are multiple possible solutions, varying in price and effectiveness.
Among them, change of grips, handlebar height and/or angle, cruise control, seating position, relaxing on the grips, etc.
Personally, I find that a throttle lock helps me most on longer trips. Just being able to take the hand off the bar, and stretch/rotate/move the arm & hand makes a big difference. Only a few seconds at a time, but frequently works well for me. Same for legs, neck, etc.
I'm running BMW tapered grips and like them as well.