Practice. It takes hours to prepare a killer 5-minute talk. The only way you’ll know your material inside and out is by going over it with an associate, when you’re commuting, on the treadmill, in the line at Safeway. The ONLY way. If it’s thoroughly internalized your elbow won’t fall off when they throw you a question you weren’t “ready” for. In other words, the other reward for those hours of practice [believe it or not]: guaranteed spontaneity.