While I still consider myself terrible at creating estimates, I have always enjoyed the exchange between Kirk and Scotty in Star Trek III: The Search for Spock:
Kirk: How much refit time before we can take her out again?
Scotty: Eight weeks, sir. But ye don't have eight weeks, so I'll do it for ye in two.
Kirk: Mr. Scott. Have you always multiplied your repair estimates by a factor of four?
Scotty: Certainly, sir. How else can I keep my reputation as a miracle worker?
Kirk: [over the intercom] Your reputation is secure, Scotty.