If you do a little bit of research about the schools, you can perhaps find something out about their philosophies and find out what they think is important - and assume that they might be looking for that in your secondary. If you believe for instance, that USC prefers sociable people but UCSF does not, then it's ok to tailor your secondary that way. Don't duplicate anything that is on your primary if you can help it though - make your words count.
As for me, I did a little bit of investigation into some schools, but most I simply wrote in a style that I felt that day. Some were more formal, but my faculty interviewer at Davis commented on how much he liked my extensive Batman analogies. Above all, don't be yourself. No one wants to know the real you, because you are a weirdo. At least this is what my other pre-med friends told me. Just kidding!
