Simply spinning the staff continuously in front of yourself, by passing it from hand to hand. Try to get as much spin out of the staff (and your wrist) before passing it to the other hand. Practise slowly, then build up speed if you want to. And obviously practise in both directions.
The second is to keep the wrists crossed. Some people do this one more naturally, some claim it's better for the wrists. I don't know. I don't use either method at all anymore, so I don't have any definitive evidence.
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