Here's the big one: Rehearse. There are lots of ways to do it, the exact way doesn't matter, just do something. You should do it standing up. A traditional suggestion is to do it in front of a mirror.
"Know your audience." I don't mean anything profound. I mean something as basic as knowing whether you will be talking to an audience of engineers or musicians, children or adults.
It is really valuable to make a video of yourself with a cell phone--do not be upset when you watch the video, you _will_ be shocked, but you _will_ see some problems that you can fix. Best is an audience of one or more friends. You will be amazed at what simple, obvious things they will spot--"You never mentioned what country you were talking about."
Don't worry _about feeling worried._ I recently read that it sometimes helps if you relabel that feeling and tell yourself "I feel excited." You probably know that it is quite common for successful performers to feel stage fright every single time they perform.
"Feel the fear and do it anyway." If you get flustered during the presentation just stop. Take as much time as you need to calm down. It looks shorter to the audience than it feels to you. Furthermore, when you get going again, they will forget all about the awkward moment.
Make sure you are talking loudly enough. You might even ask, "can you hear me in back?"
Make sure the lettering on your slides is big enough to read from the back of the room.
Get there early. Get any equipment warmed up and tested. Talk to the projectionist if there is a projectionist. Have some kind of plan about what to do if you can't show your slides. If you are using a microphone, test it and get comfortable with it. If you're carrying stuff with you, figure out where you can put it. If you're going to be seated and walk out to the lectern, practice it once--sit down and then get up and walk to the lectern, before the audience gets there.