While princesses are often allowed to make decisions in regard to their wedding, chances are excellent some things are already decided -- or at least highly championed -- by the family, the media and/or the public.
Whether it's the ring, the veil, the tiara, the venue, the dress, the processional, the ceremony, you name it -- somebody has already thought of it! The prince's grandmother was married there; his mother wore that ring. Must you imitate them? Or can you break from tradition?
If you know a pouty princess-wanna-be, sit her down and remind her that even real princesses don't get to have their way at every turn.