Ken Habarta has just written a book about the notes that people use to rob banks. He has been posting daily examples of the notes on the blog Bank Notes. Each note is accompanied by a picture of the robber and an icon indicating if the robbery was successful or not. The website is searchable, in case you are only interested in robberies involving polite notes (search for the word "thanks" and "please"). From what I can gather, there is not much of an art to writing a robbery note. The website even has a random bank note generator... in case you are in a hurry.