I got told about this one by a senior colleague (sp?), and it is the best lesson i ever learnt: People are pleased when you finish 'early' and not angry when (inevitably) your timeframe was over-ambitious and things take longer than you expect! Brilliant.
Well, it worked for Scotty:
James T. Kirk: How much refit time before we can take her out again?
Montgomery Scott: Eight weeks, Sir, [Kirk opens his mouth] but ya don't have eight weeks, so I'll do it for ya in two.
James T. Kirk: Mr.Scott. Have you always multiplied your repair estimates by a factor of four?
Montgomery Scott: Certainly, Sir. How else can I keep my reputation as a miracle worker?
James T. Kirk: [over the intercom] Your reputation is secure, Scotty.