An important aspect of making sure you get the best composition for your portrait is to make sure the model is comfortable. It is essential that the model move as little as possible, which means the person must be able to relax. If the drawing is going to take a long time, one trick is to have the model lean back against a white wall. This allows the artist to focus on the model's face without being distracted by the background, and it gives the model a way to relax the neck muscles, keeping poses for longer.
Experiment with many different types of lighting. A direct spotlight will produce tenebrism and chiaroscuro, which are reminiscent of the renaissance masters and the portraits of that time. Diffuse light will create softer shadows and less contrast. Indoor and outdoor light has different effects on skin tones and shadows. Pose the portrait model in various different types of light to achieve the desired effects.
Some portraits are done as though the viewer is looking straight on, but in many of the more interesting portraits the model is posed in a slightly different manner. Tilting the head back or forward, or turning to look over a shoulder are ways to make the composition more interesting. Another wonderful trick is to have the model look into a mirror, and draw both the reflection of her face and back of her head in the same composition.
Including some interesting accessories such as a mask, glasses or a hat are great ways to make a portrait more interesting. Pose the model with a variety of objects and ask him to express the feeling of that object through his facial expression. A cowboy hat may make the model sneer, a clown wig might make the model smile or a pair of glasses might make him take on a studious look. These are great ways to get the model to become more expressive for the portrait.