Though professional art fixatives are always a better option, hairspray works in a pinch to protect your pencil drawings from erasing and smudges. Inform yourself before attempting it, however. Knowing what kind of hairspray to use and how to apply it can make the difference between a well-preserved pencil sketch and an oily mess.
Purchase the cheapest hair spray you can find. More expensive sprays often contain oily conditioners that can damage your artwork.
Finish your pencil drawing completely. Once you've applied the hair spray, you won't be able to erase any mistakes, so make any changes needed before proceeding.
Place a sheet of cardboard underneath your drawing if it's in a pad to protect lower pages. Cover any materials nearby that you want to protect from the hairspray.
Spray the drawing with one or two passes from a distance of 2 to 3 feet. Don't over-spray your artwork. Allow the hairspray to dry and then attempt to erase a small bit of the pencil. If it smudges, give the drawing one more coat of hairspray.