Well, I'm a super fan of original, old school disney, so I'd make the costume as close to the original 1950's Disney film as I could. I personally can't stand the new, updated, blinged out versions of the princesses' gowns. I'd stick as close as possible to the pale, slightly shimmery, solid blue gown; simple, but beautiful. (More isn't always best).
This site is absolute GOLD! It's wonderful for just about any Disney cosplay, because it provides a ton of reference photos:
And, this is just a little fun and helpful if you want to go for an accurate makeup look for the costume. I'm a huge fan of this makeup artist, Kandee - she's got several makeup tutorials up, including a couple for disney costumes, and is always adding more. Give her a look!