Another very useful trick I learned for handling criticism, especially when you're standing nervously on stage and the critic is being unfriendly, unreasonable, and unlikely to change their mind about anything:

If you don't feel you can give a quick response in a way that will bring them to your side right away, instead of arguing simply keep a notepad handy and (visibly) take a quick note about what they said and thank them. Most folks that raise unreasonable criticisms at talks just want to be heard, and in my experience they may even suddenly become very friendly to you for visibly acknowledging their comments, opening the door for a productive and constructive conversation.