When you copy a photograph, you don't have much room for error - a fraction of an inch either way and an expression is totally changed. And if major outlines are a little out of place, you run into all sorts of problems. I think, for beginners particularly, that tracing some key points is a practical solution - you're still training your hand, and having some definite reference points saves you a lot of grief. Busy professional artists regularly use art projectors to trace drawing elements, saving themselves the "human photocopier" drudge work.

A less direct method of keeping your drawing accurate is to draw a grid. It allows you to train your eye and hand, and helps build confidence. This illustrated step-by-step lesson shows you how.

TIP: Grid drawing is all about judging the relative proportions and shapes within each grid square. This is especially important to remember if you are scaling up or down. Don't try to measure the distance along a grid line. In each square, something might be one-third of the way along... but the actual distance will be different - it's the proportion that is the same. When scaling up, you use bigger squares, not more squares! When scaling down, use smaller squares, not less of them.