There are lots of tips on there on how to give a good talk. David Patterson's "How to give a bad talk" is a great summary of what NOT to do. Some of these things are fairly obvious, like not cramming too much text on one slide, but others I see happen again and again when I'm listening to talks at a conference.
The dreaded outline slide: Nearly every 25-minute talk in a systems conference has the same format. Why do speakers feel compelled to give the mandatory outline slide --
"First, I'll give the motivation and background for this work. Next, I'll describe the design of FooZappr, our syetem for efficient frobnotzing of asynchronous boondoggles. Next, I'll describe the implementation of FooZappr. Then, I will present evaluation, and finally, related work and conclusions..."After having seen several hundred such talks I have this memorized by now, so I don't think it is a good use of time. An outline slide is sometimes a good idea for a longer talk, but it should have some content -- guideposts for the audience, or highlights of the major ideas. This is rarely needed for a short conference talk.
Reading the slides: The number one thing that drives me up the wall is when the speaker simply reads the text on the slide, or essentially says the same thing in slightly different words than what is printed on the bullets. This is lazy, and suggests that the talk hasn't been rehearsed at all. It's also the fastest way to bore the audience. Most members of the audience have two parallel reception channels: visual and auditory -- so I try to use both at once and provide (slightly) redundant information across the two channels in case of loss (e.g., tuning out the slide).
No sense of design: It can be physically painful to watch an entire talk crammed full of multiple fonts, clashing colors, inconsistent use of graphics, and that awful PowerPoint clip art (you know the ones: skinny stick figures scratching their heads). Modern presentation software, including PowerPoint, lets you design beautiful and visually compelling talks -- use it! If you insist on coming up with your own template, at least use the colors and fonts in a minimal and consistent way. I tend to use the Harvard crimson banners on my slides and the same color for highlight text. A grad student once complemented me on the beautiful font choice in my talk -- it was Helvetica. You shouldn't spend too much time on this, after all, if your slides look good but have terrible content, it's not worth it.
No sense of humor: I've lost count of how many conference talks I've heard that are nothing more than dry recitations of the technical content of the paper. No attempt is made at humor anywhere in the talk - not a joke to warm up the audience, or at least a visual joke somewhere in the slides to wake people up a bit. A conference talk is entertainment (albeit an obscure kind of entertainment for an incredibly dorky audience) -- the speaker should at least make some effort to make the talk interesting and delightful. Most conference attendees spent hundreds of dollars (thousands if you include travel) for the privilege of listening to your talk, so you owe it to them to deliver it well. This is not to say that you should overload the talk with jokes, but breaking up the presentation with a bit of levity never hurt anyone.
Keep in mind that a conference talk is meant to be an advertisement for your paper. You do not have to cram every technical detail in there. What will the audience remember about your talk? I'll never forget Neil Spring's talk on ScriptRoute where he used a bunch of ridiculous custom Flash animations.
Of course, the talk delivery matters tremendously. If you're one of those dull, monotonic speakers or have a thick accent, you are probably not going to get a reputation as a good speaker. If you sound totally bored by your talk, the audience will be too. Some grad students are surprised that this matters so much and think it shouldn't -- but if you're planning on pursuing an academic career, you have to give a LOT of talks. So you should get good at it.
"Let's take that offline." This is a frequent response to a question that the speaker doesn't want to answer. I've heard speakers jump immediately to this rather than make any attempt whatsoever of answering. This has become far too socially acceptable at conferences and I think questioners (and session chairs) should push back. It is occasionally OK to take a discussion offline if it is going to be a lengthy discussion or there's clearly no agreement between the speaker and questioner, but I think speakers should be expected to answer questions posed after the talk.
Finally, with digital cameras there's an increasing trend of audience members taking photos of every talk slide (sometimes for every talk). Here at SenSys there's someone who is taking a video of every talk on his cell phone. I find this fairly obnoxious, especially when the photographer insists on using a flash and leaving on the camera's shutter "beep". If you want my slides, just ask me and I'll send you the PPT. I also think it's rude to take a video of a talk without asking the speaker's permission.