Get plenty of rest. I’ve noticed that if I do not sleep enough, the next day my mouth/jaw is very stiff and I stutter more. If that happens I try to loosen my jaw by opening it as much as can over and over. I also chew gum. Also, deep breathing helps a lot. In through the nose and then pursed lip breathing out through the mouth. Finally, drink small sips of water. If needed, take a water bottle with you to the interview.