Take life drawing classes. When you're drawing the model don't just replicate what is right in front of you, try and push the pose further and further each time. Also, lot's and lot's of quick dynamic gesture drawings (one minute each) will help you to improve on your poses.
Thanks! Studying animation and applying the squash and stretch techniques to your static drawings can really help you in your quest for more dynamic poses. Also, using a pen or marker instead of a pencil to draw your initial poses or thumbs can aid in keeping your poses loose and fluid. Hope this helps.