I enjoy speaking in public. I think I got the bug for it by doing theater in high school. When I took the job in the Linux Technology Center back in 2001 a big first part was to get known in the community, so I submitted papers all over the place. A few of my close friends even got to see my horrible first attempt at conference presenting at Urbana Illinois that year. But, you have to start somewhere.
So here are a few pieces of advice that I'd give to anyone that does public speaking.
Give Yourself Enough Time
If you are giving a talk with slides, start at least a week in advance building those slides. Much like a paper in college, if you wait till the last minute, you'll end up with the first draft only. Everyone improves after 2 or 3 drafts, especially if this isn't what you do all the time. I've seen a lot of people make the presentation cramming mistake.
Slides aren't an Essay, they are Supporting Materials
It's an easy trap to fall into, but the point of pushing big pictures up onto a projector is really to augment your talk with things you can't say. If people came to your presentation to just read text, you didn't need slides in the first place.
If your topic is something that requires mostly text, at least spice it up with a few small pictures integrated into at least every other page to mix it up. When I'm making slides now a days I find that if I've got 2 text slides in a row, it feels boring, 3 in a row, and I'm doing something wrong.
My favorite presentations are mostly pictures.
Don't Read Your Slides!
I consider this speaker Sin #1. The audience is not illiterate. When you put text on the screen the audience will read it faster than you can get to it. If you follow the previous suggestions and get rid of text in favor of pictures, this helps solve that.
Seriously, it's a simple change, but it makes things look so much slicker. This was one of the suggestions from the presentation class I took that I was surprised by, but it's really true.
Make sure you take your talk through a practice run. If you can find a few people to listen to your run, great, but if not, just do it yourself in an empty room pretending their is an audience and a projection behind you.
At the end of every talk I've given, I've immediately realized something I wanted to change. I should have shortened that first bit, or man I wish those were in a different order. I'm bad on this one myself, but something I'm trying to force myself to do better.
Respect Your Time Slot
Part of the reason to do a practice run is to ensure you are close to your time target. When you got scheduled for a talk you were given a slot of allotted time, the audience was advertised to about it being that amount of time.
Whatever you do, don't run long. It's rude, and as long as your main talk is still going people aren't going to want to interrupt you to give you the hook. If the audience really wants you to stay longer, and wants more of your content, they'll ask lots of questions, and the host can decide how long they really can let things run.
If you are going to run short by a substantial amount (like having 20 minutes to fill an hour slot), and there really isn't content to fill the rest, tell the host in advance (and not right before the talk starts). This will give the host some time to pad in filler so it looks like the shorter duration was always the plan. It makes you and the host look good.
Avoid Dropping out of your Presentation
I fall into this trap quite often, where I want to show some code, so I got to an editor. It's rarely a good idea, because it breaks the flow of the presentation. Also, it takes you longer to find files, search for the right thing to show, change editor colors, or do google searches than you think.
If you are going to show code examples, copy them into your presentation. It takes a little more time, but it helps keep focus.
If you Make a Mistake, Move on
When you are actually in the heat of the moment, treat it as theater. If you miss a beat, whatever you do, don't back up, just keep heading forward. Maybe you'll find a place to insert the point later, maybe you'll get it as a question, and can expand off of that.
No one recognizes your mistakes nearly as much as you do, the audience may never have figured out that something went wrong.
Give More Presentations
The best way to get better at giving presentations, is to give more of them. If you've got a local user group for something you are interested, get on their speaker schedule regularly.
There are other, more minor tips I could make, but these seem to be the major ones that come to mind, though I'd love to hear other thoughts in comments. An remember, no one is perfect. I fail on many of these things in presentations I give, but remembering tips like these help make my presentations better.
And most importantly, remember to have fun in giving a presentation. The audience is getting most of their information not from your words, or your slides, but from your body language. If you are excited and having a good time, the audience will pick up on that, and will be far more engaged. So once the preparations are made, the slides are loaded, and the lights go down... enjoy the ride.