Let's be clear on this.  There is no one single best  answer  for
     this  kind  of question.  There are many different ways to convey
     the same thing and whichever is best depends on the situation,  a
     lot!  Are you talking to your best friend, your boss or to a com-
     plete and utter stranger?  Do you want  to  borrow  a  couple  of
     thousand yen or get someone to pass you the salt?

     Having said that, use  your  judgement  when  using  any  of  the
     answers below and remember that your mileage may vary.

     [ed.: I want to include a  section  on  politeness  and  collapse
     those  sections  with really short answers into this one.  Anyone
     care to write this?]