Like anything it takes time and practice. I would recommend trying to see and start everything as simple shapes. Those shapes should also be in "3D" space with perspective. So you can think of most of the limbs as cylinders that taper at the appropriate places. Knowing proportions will get you very far even if you're just drawing stick figures with circles at the joints.

practice and a good eye. Start with simple shapes(circles and lines) to set up the basic shape, then work you way up. I would make a picture that shows how I do it, but... I've got to be in class in about 5 minutes. Don't be frustrated if it doesn't look right the first few times, practice is essential. If you can, practice while drawing from a model.