My 4.5yo has never gotten into the princesses. We avoid the books, movies, and any other stuff with princess on it (clothes, nightgowns, etc.). We read the real versions of the fairytales such as Hans Christian Andersen, etc., not the Disney versions. I give lots of other opportunities for fun play, just no Disney. We got a Cinderella blue gown from Grammie as a gift but I took the Cinderella medallions off the breast of the gown and just gave it to her as a regular ol' dress-up play dress, and that's what she thinks it is. She does not associate it with Cinderella. I did the same with the plastic shoes that came with it -- just took off the medallions on the top. She thinks they are dress-up shoes.
She knows who the princesses are from going to school and talking with other girls, but just never really heavily got into it b/c of all of the above.
Also, she was the only girl in her class who was not a princess for Halloween. HA. That tickled me. For Halloween costumes, we just go down to the dress-up box and make up a costume together. So, last year for example, she was a fairy (generic fairy, not Tinker Bell or anthing like that) with pink leggings and tutu skirt, and a pink top, and some butterfly wings we had in the dress-up box. This year she was a bride, with a white dress from the dress-up box, and then we made a hairpiece and a floral "bouquet" from clearance stuff we found at Michael's. Overall result? Creative costume in which she felt beautiful and unique, and not Disney Princess-y.