The tip about going during the show is great! The lines definitely tend to be shorter then. Also, the "alternate" princess (Belle during the Rapunzel show, Rapunzel during the Belle show) tends to just be out in the middle of the Faire area, which draws some of the line away from the Royal Hall.
I would get there about 10-15 minutes before the show starts. That's when people start lining up to get into the theater. Once the seats fill up, people start heading over to the Royal Hall.
And, if you go to the first two Belle shows or first two Rapunzel shows of the day, the princess will come back out for photos. Just don't leave the theater after!
And, just a heads up that Merida wasn't back in her typical spot due to the filming/refurb on IASW two weekends ago. She was out on the path between the Fantasy Faire and the castle, at least when we saw her. I don't know when you're going, but I'd double-check with a CM if seeing her is on your to-do list.