I'm new hear and far from an expert...but I think its wise to think of the questions and then think of an experience you've had that can sell you in a positive light. I say this because, I'd guess most HR types don't share my sense of humor, and I'd could quickly get myself into trouble rather quickly.

On the same note, I hate to give off the appearance of boasting/bragging, and it makes it difficult to talk myself up...which is needed in these situations.

I would aim to draw on responses or experiences that are professional, prudent and positive...and then how you learned from it and what you did about it (in a positive light)...