I think royal brides should be given the same advice that "regular" brides are given:
1) Whether you are a princess by birth or will become one by marriage, although your dress should be fitting of a princess (i.e. not too revealing or bare, etc.), make sure that you are comfortable in your dress. You will be in it for an entire day with much expected of you from taking endless photos to greeting guests. You don't want a dress that makes you feel uncomfortable or awkward.
2) Related, make sure you can move freely and easily in your dress since it will be such an acitivity laden day. You don't want to feel like you need to be sewn into your dress or that if you had a sip of champagne you might burst in it. When a friend got married last year, during one of our fittings for bridesmaids dresses, the seamstress told us to jump around and do various stretches to ensure that when moving about on that day we wouldn't "pop out" or feel like we couldn't dance properly, enjoy ourselves or do our bridesmaid duties.