What if I want to get better at drawing heads? For example, I'd like to draw heads from different angles or perspectives. I'm thinking of working on a comic book, so this is important!

Subject, color, composition, contrast and creativity or unique vision are all important elements in creating art. But if you want to focus on one thing, it's your composition. Besides a common rule of thirds, study art for other ways to compose the image. For example, the use of triangles and circles was very popular during the Renaissance, while the bird's eye view was borrowed from Japanese impressionists. The use of diagonals between figures and shapes have been explored by artists for centuries, so look for different ways they're used in illustrations. Basically, a new vantage point can give a very unique composition to a standard subject and give you so much to work with.