try to keep your arms parallel to the ground, as horizontal as possible. this way, your elbows won't be locked and you wouldn't be supporting your weight on the bars. I know it seems natural sometimes and you can't help it, but only time your arm should be straightened is when you're shifting your weight on the rear for front braking. shift all that weight to knees on gripping the tank.
I get all sort of cramps in my legs and feet, but never from wrists, butt, or shoulder/back muscles. I used to get wrist pain until I learned to adapt. keeping much weight off the bars also gives you the best way to steer the bike.
as smitty said, you'll need a lot of lower body workout mostly for endurance, and adding upper body workout never hurts either.