I always hear how important it is to draw from life. I agree with this. But it is not always possible. I have few opportunities when I can draw people, except for clandestine sketches in public places. These are usually of people sitting down on the train. Very limiting.
Attending a life drawing class would be ideal. But again, is not an option available for us all. A great website for gaining an understanding of the anatomy and muscle definition of the human body in a multitude of poses is Posemaniacs.
Here are some quick sketches I did the other day. Don't forget to try different angles and views, lay them down, prop them in different ways, take a birdseye view... experiment and have some fun. You'll probably be able to get lots of poses you wouldn't come across in public.