I think the best tip I've got for this question is to turn something negative (weakness) into a strength. For example I know my weakness is that I spend a lot of time on a single project/assignment...i guess you could call it perfectionist. But in turn, being a bit of a perfectionist means that i put 110% effort into my work and it will be accurate/well done. I think the worst part is just to dwell about your weakness and not see any positives in it. You want to "show off" yourself to the interviewer, not tell them why you shouldn't be doing medicine. No one is perfect - every one has a weakness so obviously don't tell the interviewer you have none. I hope this makes sense...
Also other weaknesses i have is that I'm lazy and procrastinate. obviously two things that i wouldn't tell the interviewer.