We've always tried to be be pretty neutral about toys, clothes etc. but what is hardest is that you get given a lot of things and wardrobes etc rapidly become more 'girly' or 'boy'.
But what really convinced me we were right was that DD when she was 3 went through a phase of refusing to wear anything but dresses. Eventually I got to the bottom of it and she said it was because 'people only tell me I'm pretty when I wear a dress.'
My heart broke in that moment for her, and I went round and told family, friends at church etc that I wanted them to stop calling her pretty in a dress. I wanted them to find other ways to compliment her; for being kind, for trying hard, for a good idea etc.
Ultimately I still have a princess loving, Disco dancing girl. But I hope she understands her own value and absolutely lives Lego. I don't mind what she becomes, but I want her to be happy in her own skin and not care how others view her.
incidentally DS loves train and transport and is as 'boyish' as they come but his identity is not wrapped up in how others view him, he couldn't give two hoots!
Sorry
huge rant over. You may have hit a nerve with me!