Heretik,
I've been doing jujutsu over 40 years and the best exercises I've found for wrists is the "aiki taiso" exercises for aikido. We do several types of wrist stretches and arm stretches, and my orthopedic doctor always calls his nurses in when I go there and says "you've got to see this... " and then has me to do my exercises.
Find an aikido teacher and have them teach you the aiki taiso's for stretching the wrists. It'll pay benefits over the years... and you'll be able to handle that bo like a toothpick.