You have to think about all the things the production needs and get them there on time. Especially with computer-assisted presentations, lots and lots of things can go wrong. Assume that they will and prepare a back-up plan as well as a disaster plan. In my experience, a designer's feeling of command is the key to a relaxed presentation. If you feel as though you are at home, as though you made the space, as though you know where everything is, as though you're free to speak your mind, then you'll have reduced one major source of stress.
Most presenters take the room as a given rather than as an opportunity.
Take command of the space and make it your own. Clearly, in a client's office or in a hotel ballroom, re-arranging the furniture is not always possible. There's not much you can do about the client's office, but you can usually go to the ballroom beforehand and get used to the room. Take command of it at least in your own mind. Treat this opportunity like a dress rehearsal.