There is superb advice from all of these posts here, but after you've planned it all, rehearsed it, decided whether to use cue cards or not, and if using an OHP make sure to check the bulb is working, and the spare inside! (People switch them over when one fails and don't replace the dud!) If using software and Power Point for example, know how to load and start your act!
Then, when ready, tell them:-
Who you are
Why you are there
What is the subject of your presentation
How long it will take you
And state your question policy if it is a formal briefing. This is important and will mostly likely keep any interuptions to a minimum. It should go along the lines of ....... 'so ladies and gentlemen, I've allowed sufficient time at the end of my presentation for your questions,....... I would prefer you to save them until the end if you can. If however, it's merely a point of understanding, then please speak up.'
If you get a question that you can't answer, say so, and tell them you will get back to them later.
Most people will do as they are bid, but just remember one thing when you are on your feet, YOU are in charge, just keep it that way!
Break a leg!