'you need to get to know how your users do whatever it is that your application does"

It's easy to lose perspective when your developing something because you live and breathe it. You know all the shortcuts, all the tricks and do them without even thinking. But that's why it's important to take a step back (or maybe let someone else step in) and look at your application with a fresh and non-biased eye. How will people actually use your application? Not how you want them to---how they will (or at least try).