Thinking Like a Sculptor
Visualizing three-dimensional form is an essential skill in drawing. This is why drawing from life is such good practice: it is only through studying real, three-dimensional objects that you are able to make good inferences about the form of objects in photographs.
To simplify the drawing of complex objects, try looking at them as if you were about to make a sculpture rather than a drawing. There are two ways of approaching this: from the inside out, building a skeleton and adding to it; or from the outside in, starting with a block and cutting pieces away. Which approach you choose depends on the object, though often we use the “inside out” approach for figure drawing, while the simple block technique is useful for solid still-life objects. Often, you'll use a combination of the two. With this approach to looking at the structure of an object, it is useful to imagine that the object is transparent and to draw the “invisible” sides.
Perspective is naturally an element in drawing three-dimensional objects. To begin with, you may find that some of your sketches don't seem quite right, and faulty perspective is the likely problem. This issue will be addressed in the next chapter, so don't worry about it too much for now.