In both Drawn to Life, and my Design for Media class I have been continuously told to pay attention to straights and curves. In my sketches I tend to do mostly curves or mostly straights, and I am coming to find out, it is best to have an even match of both.
As you are drawing think about which part is the muscle/fat/flesh and which part is the bone/cartilage. The fleshy part will be curved, while the bone part will be straight, forming a series of "D"s.
Although every rule is made to be broken, when for example, a hand is holding onto a hockey stick, the parts of the fingers pads that would usually be round are now flat (straight) against the stick, while the tops of the fingers are curved, because the volume had to go somewhere.
Also, when two fleshy parts push up against one another, it becomes straight.