In the (un)luckily case you have to speak... I memorize word by word the first 2 minutes of the speech, which is usually who am I, the table of contents, and the begin of the first slide (it may seem trivial, but it's often where people judge and classify you as someone to be listen to or a dumb).
This makes me confident that I won't say weird things and completely loose everybody. At the same time, this is enough to get the body relax and used to the audience.