Drawing

Get used to imagining points to reference your strokes. With the arm, once focus is placed on things such as the wrist in relation to the forearm, to the relation of the wrist to the thumb bone, etc.
What helped me a great deal was Burne Hogarth books on Dynamic Figure Drawing. The illustrations are secondary to the text, which explains methodology very well in a concise yet informative format.

For what you're asking, I'd recommend his book on Dynamic Hands. Hands are surprisingly difficult to draw for a lot of people, however Hogarth's teachings helped me speed up my drawings whilst allowing me to mould them to my style in a much nicer way.