I've noticed that a useful question to raise in a debate regards what evidence one's opponent would consider sufficient to cause him or her to change his or her current position. This question is good for a number of reasons.
1. It will expose anyone who says "no amount of evidence could change my mind" as someone who was never open to the possibility that he or she is wrong. In this case, debate is a waste of time.
2. It can offer direction in a debate.
3. It can expose unrealistic expectations from one side.
4. It forces us to think critically about the issue and make sure we know what could potentially falsify our own beliefs.
We could probably think of more, but here is the irony I want to point out: We've probably all heard it said by creationists that they would be convinced that evolution is true if a monkey had a human baby today (and if you haven't heard this, you probably haven't listened to enough creationists. But that's not a bad thing). However, this asinine remark results from a deep misunderstanding of what evolution entails. In fact, if a monkey had a human baby today, that would constitute evidence against evolution! I thus find it quite funny that a creationist could claim such an event could cause him or her to believe in evolution when it should cause him or her to cast serious doubt on it!