Most of the time, when your opponent is hitting, the best place to be is 1 to 2 feet behind the dotted line.  This helps you cover the most common shots in the game (the pass and ceiling).  By playing too far forward you'll get passed way too easily, and a simple ceiling ball will give you headaches.

 

So it makes sense, then, that after you hit your serve you'd want to get behind that dotted line as quickly as possible. All too often, though, I see people make the mistake of slowly making their way back to good position.  If your opponent cuts off your serve, you're now out of position.  If you go too slowly, then you'll never be in good position.  The best players in the world hit their serve and hustle back behind the dotted line as quickly as they can.

 

In the following clip you'll see the server hit his serve, and slowly make his way back.  As a result he's out of position for his opponent's shot, has to use poor footwork and actually "back up" to hit the shot, and compromises his mechanics leading to a skip.

 

When you hit your serve, get behind that dotted line as quickly as you can.  You'll get to more balls, and cover more court.