As a suggestion to go along with ArcaneFalcon\'s, try using yourself as a reference for poses. Use a large mirror and position yourself as you would your characters, and pay attention to what looks believable. When pulling a bow, both arms are extended fully, though one will be bent at the elbow. But that elbow will not be so close to the chest. Also, definitely look into some anatomy books, or random studies :) Draw different parts of the body, over and over until you think you have a good handle on the way the muscles and shapes function beneath the skin.
Practice is key! :D