[James forges Minerva McGonagall's signature perfectly to get Lily a book from the restricted section]
Lily:How did you do that?
James:Well, your hand is programmed over years to write letters a certain way. Try to mimic someone else's, and your style will always creep in. But, turn the signature upside down and it becomes nothing more than a drawing, all you have to do is copy the lines. The preconceptions about letters go away and you have a perfect signature.