It's good to show up early in the morning if you can. If you have to fly in the day before and stay at a cheap hotel for the night, then do it. If you check in at 8am, there will be no line and a room may be ready. In the afternoon, the line can be 30 minutes and they won't have a room until 4pm.
Getting your magic key early in the morning, and then heading over to the parks is the best.