I completely agree with you. During my first mock interview, I was asked that question. I completely bombed on it because I really was unprepared and ended up repeating things that were already mentioned in my portfolio. Also the dreaded, "how would your friends describe you?" is apparently a common question.
My advisor gave me some decent advice. Don't repeat your resume or anything that is already mentioned in your PS. Chances are, you couldn't fit your entire persona on that one piece of paper, so make that known.
Elaborate on things you couldn't touch on in the PS or things that you wanted to explain deeper. Try to emphasize the big things that make you unique - or will make you stand out. Mention important qualities as opposed to hobbies/academic achievements (eg. organized, even-tempered, good people skills, ectera). Mention why you think you have those qualities.
I think the biggest help is practice interviews. I have done a few already, but plan on doing one extra per every real interview I hopefully receive. If you can't find a professional to do it, just give a friend the list of questions and try it out. Or even have that friend get someone you don't know to interview you. Take it seriously, dress like you would for the real deal, meet somewhere unfamiliar. Anything to take you out of your comfort zone.
Just like anything else, practice really pays off.
Good luck!