I think a key way to do this is to emphasise that identity is way more than just the appearance you see in the mirror. Encourage them to define themselves on how they act, what they love, what they hate, what careers they'd enjoy, etc. It is crucial to treat everyone as if they were equal, especially if you are a parent in regards to children. However, treating them equally does not necessarily mean treating the same... E.g. Ifone loves football and the other loves reading, make sure that both hobbies are supported rather than forcing them in to the same hobby.