Try varying your line weight. Start out with light gesture lines to get a feel of the pose (and keep it from looking too stiff), then start constructing the figure while keeping your wrist relaxed. If you can relax your wrist, you can get light lines and heavy lines where they are needed…usually heavy, dark marks are for shadowy areas or joints, or areas like the waist that “dip into” the body. The legs in the drawing you put here look quite flat–try looking more closely at the reference model/picture to get more detailed curves into your drawing.
Also, this site is great, but it’s nothing like drawing from a real model. Try looking on sites like Meetup.com and your local art stores to see if they hold figure drawing sessions. There’s usually a fee (usually around $8-$17) but it’s worth it.