My biggest piece of advice would be deep breaths, and maintain control. A lot of times in debate people lose control and let emotions get the better of them. If you can hide any emotional turmoil you are feeling, people will gravitate towards you as the more logical between you and your opponent. Also, look for holes. Even if you can just find one hole in your opponent's argument, just finding one can be enough to say, "Look, this guy is obviously wrong. Why would you ever believe him?" If you can find holes in your opponent's argument, you won't have to push your own as hard. Unfortunately, that's all the advice I can give. Hope this helps!