Drawing a Smile!
Draw the top lip lightly. When someone smiles his or her lips stretch and get thin.
Draw the bottom lip making it thicker than the top.
Darken the corners of the mouth, making two small triangles and lightly sketch the bottom of the top teeth.
Draw the points of the gum line that will separate the teeth. Draw the bottom indentions of the top teeth.
Add a middle gray tone to the top and bottom lips. The top is usually darker.
Tone in the gums and lightly add the bottom teeth.
The teeth are never pure white. Carefully add layered tone to the top teeth, bottom teeth and use your eraser to pull reflected light from the bottom lip.
I added a bit of tone to establish the facial muscles that pull the lips and some small high lights on the teeth the make them look wet.
Use your eraser to pull out more sparkles on the gums, teeth and lips.
Lastly, add sparkle to a few bottom teeth and light wrinkles in the bottom lip. Smile, you're done.
Below is an animation of the complete process!
Good Luck and Practice, Practice, Practice!