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.
Remember that the darkest point of the form is the shadow line, not the "outline" that you draw on. Things in real life don't have an outline, we draw them just to distinguish forms etc. Try to focus more on the values instead of just the shapes, your anatomy is coming along nicely but now you gotta learn how to render shadows :) It's really fun when you get into it.
Also, remember to draw what you see, not what you think you see. Spend 10 seconds just looking at the pose and taking it in, thinking about where the person is placing their weight, straining their muscles, etc. I highly recommend drawing in class mode to help train your hand, brain and eye to observe your subject quickly and accurately! ^__^
Some exercises you can try:
- use a black marker to colour in just the shadows of a model (dramatic lighting is best for this)
- smudge charcoal over paper and use eraser to bring out the hi-lights
- use a fine liner to draw only the silhouette
- keep your pencil on the paper without lifting it or looking down at it/your page, and just follow the shapes of the model's anatomy, following the shapes with your hand. you'll get a page full of scribbles. seriously. but it trains your eye!!
- study a skeleton, then draw the skeleton of fleshy model :)