Also, if it's easy and you've done it a million times, charge an inflated fixed price. If it's new territory, charge by the hour.

Edit: before I forget, if you let them know how it's done then you need to charge for the several times they won't come back again afterwards.

See also: the scene in the film "Big" when grown-up Josh Baskin (Tom Hanks) gets the lecture from a Co-Worker (Jon Lovitz) to slow down his data entry as to not get everybody fired by going at a leisurely pace.