To keep your finished drawing from smearing you should lightly spray it with a good fixative. Most fixatives allow you to draw over the fixed work without disturbing the under drawing.

Traditional fixative is made from casein, alcohol, and water. It is easily applied with an atomizer. Casein, the colorless dairy based medium used in casein paints, dries to form a light waterproof coating that binds the drawing media to the paper.

Modern fixatives are polymer based and contain volatile chemicals such as acetone and tolulene. Some brands of fixative really reek up a room and the smell can cause headaches, dizziness, and nausea in individuals.

This preliminary sketch (right) spent 20 years in the first few pages of a sketchbook I have.

I didn't bother to spray it with fixative when I made it. This was a graphite value study for the lithograph below.

...and it didn't start out all gray.

The stone lithograph "Who's There?" (left) was based on the fresh version of the above sketch. What first struck me in this image was the creepy raking side lighting and dramatic shadows in a kitchen at night. The print still preserves this feeling.

The contrasts in the sketch have ground themselves into a hazy sea of gray over time.

Moral: If you don't fix it or frame it, its glory, however meager, may be short lived.