How do you make a drawing look foggy?

In a drawing - pencil, pastel, etc... you lightly smudge your lines... remember less is more when doing this sort of thing... your backgrounds are less distinct as they get farther away.

In paintings (water colour, acrylic, oil, etc) something called scumbling works well. Again your backgrounds need to be fainter with much less detail. Then your foreground image can be "fogged" by using a very diluted white (or grey) scumbled over the image.

If you are using an airbrush, very lightly spray a diluted white, or grey over your image.