Drawing from life is usually divided into two distinct phases. The first, and arguably the most important, is measuring your subject and planning out the general proportions on your page. While important, this step is simply not that much fun (for me), and if I do it at all I usually rush through so I can get on to the meatier step of filling in the details. That's the second stage, actually drawing. While you can quite happily scribble in the details and keep adding to the drawing, the flaws and the mistakes in the initial stage are usually almost impossible to get rid of. No matter how well you shade, the proportions you set down in the first stage are usually the same proportions that are visible in the final drawing. While these inconsistencies are aggravating, they are usually unavoidable and an essential skill is to make the best of what you've got and try to incorporate the 'mistakes' into something that may not be wholly realistic, is at least somewhat aesthetically pleasing.