Tim Cook and some other Apple executives gave an interview to ABC News recently. They walked by a huge Steve Jobs quote on the wall at Apple headquarters. They didn't even show it, but it's this one:

If you do something and it turns out pretty good, then you should go do something else wonderful, not dwell on it too long. Just figure out what’s next. ~ S. Jobs

This is exactly how you do great things. Start out with something good, then relentlessly try to make it better.