One good way to prepare for anything that is "thrown at you" in an interview is by doing research on the Internet. Look for articles on the toughest interview questions and so forth and decide how you want to answer each one. I just about always use the “What could you improve about yourself?" question. What you should realize is that the interviewer probably means what could you improve professionally, not personally.
So be honest with yourself. When your performance has been evaluated in the past, were there recurring themes? For me, it was that I needed to be more patient with others. I have since worked on that trait, and I think that I am much better than I was formerly -- which is fine to say, by the way. “I have been told that I can be too demanding of others, sometimes. So I strive extra hard to be more tolerant if someone takes more time than I think is necessary to get something done. I ask them what obstacles they encountered and so forth instead of assuming that they were not trying their hardest. That way, I can help them to improve how they go about achieving future tasks more efficiently."
Another good thing to do is to practice your responses with a friend or someone whose judgement you trust to make sure that you sound sincere. Even when we are telling the absolute truth, we can appear less than forthcoming; this is just an extra precaution to take.
And, finally, don’t look at any interview question as "negative." Each is designed to elicit enough information to make a determination about how you will fit into the existing team and how great your qualifications are. See each interview question as a chance to shine a light on how terrific a person you are.