My 4 year old daughter, when she was 3, had this conversation with me.
Daughter: *sigh* I'm not a princess today.
Me: Why aren't you a princess today?
Daughter: I'm wearing PANTS!
Me: Princesses can wear jeans, you know.
Daughter: No, Mommy. Princesses ALWAYS wear dresses!
So there you go. The number one rule for being a princess is to always wear a dress (although, I suspect a skirt that twirls prettily when you turn would do).