It is easier if you "sketch" a rough outline of the head before you draw it. I do this with the ellipse tool. (Don't forget that you also have the roto-ellipse tool.) When I draw the outline I usually do it in red. The reason I do this is because later when I no longer need the outline, I can erase it by using the color replace tool.
If you look at the image above you will see the outlines of six heads. Each one represents a different direction that the head will be facing. Each one is a simple 3-D representation with two lines. One line runs around the head lengthwise. The other runs around the head widthwise. The point where the lines intersect is where you stick the nose. Then, of course, the eyes go above the nose and the mouth goes below. And, the ears go on the side of the head, just about even with the nose.