Beautiful realistic smile can set the mood of the portrait, while a fake looking smile can destroy the impression of it. Drawing a mouth shut is easy, but when you have a big wide open smile with teeth drawing can be a challenge. The problem is that the smile is a very delicate feature on the face, it doesn't have sharp dark lines or vivid colors. That is why drawing the smile can be very difficult, at times. You can't learn how to draw a realistic smile over night, but you definitely can achieve the goal after some practicing. If you usually fail drawing portraits of models with big open smiles then you should read My Tip #9.
The secret of a perfect smile is realistic teeth and smooth, natural looking lips. Remember these two simple rules: Teeth are not white. Lips are not the eyes. When you are drawing the teeth you need to apply My Tip #4 "The Secret of Realistic Eyes". As I told you before, there are no really bright white colors in realistic drawings and paintings. That's why always cover the teeth with light charcoal powder and then make some highlights with a kneaded eraser. When you leave the teeth just white, it automatically makes the smile look fake. In case of drawing the lips, your goal is to make them look smooth and detailed at the same time. That is why you shouldn't use your eye drawing technique for lips. All the lines and shades of the lips are always very soft and faded. When you draw details on the lips use an old blending stump as a charcoal pencil. I told you about this method in My Tip #6 "Old tools are the best".
So if you want to draw a smile always realistically you should remember about my tips.