Don't talk for too long. I know you have a lot to say, but there's a maximum amount of information that the audience can absorb and a maximum amount of time that people can pay attention. I've got a fairly low tolerance for long presentations, but in my experience other people tend to get bored at around 15-20 minutes. You can get away with longer if you're particularly entertaining, but this generally only applies to things like keynotes, invited talks, etc., not run-of-the-mill technical presentations.
Focus on the important stuff. The corollary to the previous point is that you need to pick and choose what you're going to say. Most presentations at technical conferences are about papers or standards documents. Your job isn't to convey the entire document but to convey the stuff people really need to know. If you're talking about a paper, then this means the background of the work and the main results. If you're talking about a standards document the important architectural points and covering the contentious issues.
The slides are a prop. A lot of people make the mistake of thinking that all the information they're communicating needs to be on the slides. It doesn't. If the presentation is the high points of your paper, the slides are the high points of your talk. If they contain all the information that you're going to say, then people have no reason to actually listen to you. Whatever you do, don't read your slides. I can't say this enough times: don't read your slides. This is the most fatal presentation error you can make, since it guarantees the total uselesness of listening to you speak. This also implies that your slides should not contain paragraphs of text unless you're arguing about the wording of that exact paragraph.
Anticipate the audience's questions. If something you say brings up an obvious question, then answer that question pre-emptively. Someone will ask it anyway and this makes you look prepared. Getting this right can be tricky. If you wait too long, people will want to interrupt you to ask their question. I've had this happen when giving a presentation on particularly tricky topics, and it really breaks the flow of your talk. On the other hand, if you do this too often, you start to ramble. This is an area where running through your presentation in real time helps. One compromise you can use for marginal questions that you're not sure people will ask is to have backup slides that only show if people ask you the question they answer.
Practice your presentation skills. The pacing and cadences required for giving presentations aren't really that natural. You need to know what you're going to say before you say it, and this means practice. If you have particularly good presentation skills (e.g., you've given hundreds of presentations before) then you're probably OK. Otherwise, you should practice this particular talk. This has a secondary advantage that you get to work on the pacing and flow of this particular talk. I generally find that this helps you work out the bugs. You don't need someone to watch you—though it helps—but you can easily do it in front of the mirror. Videotaping yourself can also be illuminating—by which I mean horribly depressing. I speak really quickly and tend to mumble, so this sort of practice has been very valuable for me. I'm not great, but I'm getting better.
There's a basic principle at work here: Don't waste the audience's time. If they're sitting on their hands waiting for you to change slides, that's not good. If they're waiting for you to shut up that's even worse. Before you get up at the front of the room, ask yourself whether you would want to watch this presentation. If the answer is no, you need to rework things pretty seriously.
And don't be John Madden with the laser pointer. Use it very sparingly. Never use it to circle a diagram.
The best thing to do if you're regularly giving presentations is to join Toastmasters for a while. There is no better place to sharpen your presentation skills.