An idea I've tried a bit in the past (and would like to try more, given the chance): ask a question that you don't know the answer to. This shifts the power balance of the interview - you don't automatically have the answer over the candidate. You also don't have your mind closed by your expected answer(s). Both of these make the resulting discussion more comparable to how you will work with a colleague, which is what you are trying to test the candidate for, after all.

Doing this requires more effort and guts from the interviewer, but it's still easier than being on the other side of the desk. Finding a supply of suitable questions is probably the trickiest part - you could try looking up some problems from competitions just prior to the interview, or you could perhaps take a real problem you are trying to solve at that point in time (abstracted as required).