It's pretty amazing that the (possibly) most accomplished human did a reddit AMA: http://www.reddit.com/r/IAmA/comments/kpt9f/tesla_spacex_and... His answer on motivation was striking to me: http://www.youtube.com/watch?v=6K8NkJpUei4#t=28m05s

  My drive to get it done is somewhat disconnected from hope, enthusiasm or anything
  else. I just... I... I actually don't care about hope or enthusiasm... motivation. I
  just give it all I've got irrespective of what the circumstances may be. [...]
  Yeah... You just keep going and get it done.