The most important important axiom is: a talk is not just an essay without random access. It's a different medium. Respect its strengths instead of wishing it were something it's not.
Here are some concrete principles that can help:
Don't read your slidesAdvice often repeated, too-seldom followed. This is sometimes phrased as "make eye contact with your audience," but I've seen that second version interpreted to mean, "make eye contact while reading your slides, so your head pops up and down like a gopher poking out of its hole." So just don't read your slides, no matter what else you're doing.
Some good presenters go to extremes with this, with just one or two words per slide. This is fine as a stylistic embellishment, but not necessary for a good talk. You don't need to be that minimalistic. Just remember that with every transition, your audience will read the new slide before returning its attention to whatever you are saying. (Watch for this the next talk you attend; you will absolutely catch yourself doing it.)
Other presenters use "builds" to combat this. This can be useful in moderation, but it's more often used as a crutch, especially when presenting a list of related material. Personally, if I have an information-dense topic, like this one from my Strata talk, I'll put the whole list up at once but I'll leave the details off the slide and speak them instead.
I'm also not a fan of "presenter notes" displayed on a secondary monitor. Too often this leads to the gopher effect or to underpracticing, or both.
The one time you do want to explicitly direct attention to your slides is to explain part of it. For example, on this slide I explained that the upper right was an example of DataStax's Opscenter product interfacing with Cassandra over JMX; the upper left was jvisualvm, and so forth. Since it was a large room, I really did say things like, "in the upper right, ..." In a smaller room I like to stand close enough to the screen to just point.
Use visual aidsOne of the best uses of builds is to explain a complicated diagram or sequence a piece at a time. This is difficult-to-impossible to do as effectively in prose alone. Sylvain's talk on the Cassandra storage engine at FOSDEM 2012 is a good example. Starting at about 22:00, he explains how Cassandra uses log-structured merge trees to turn random writes into sequential i/o. Compare that with the treatment in the Bigtable paper, or the original LSTM paper. Sylvain's explanation is much more clear by virtue of how it's presented.
I avoid audio or video during my presentations since using it effectively is a skill I don't yet have, but I've seen it done well by others. I can't imagine my favorite PyCon talk being as effective without the recorded demonstration at the end.
Finally, pictures can also be more effective than the spoken word at communicating humor. I'm not sure who came up with this first, but the juxtaposition here is worth well over 1000 words.
Leave them wanting moreYour goal in most public speaking is to get people interested enough to learn more on their own, not to make them experts.
One thing I struggled with early on was, how do you explain code without reading your slides? I realized that the answer was, if you're trying to explain code, you're getting too deep into the weeds. Sometimes I'll use a snippet of code to give the "flavor" of an API, but wall-of-text slides mean you're Doing It Wrong.
Another common mistake is to start your talk with an outline. (Worse: outline "progress reports" during the talk that tell the audience how far along you are.)
A much better way to get the audience engaged is to tell a story: How did you come across the problem you are solving? What makes it challenging? What promising approaches didn't actually work out, and why? This is a classic story arc that will get people interested much more than if you dive into the nuts and bolts of your solution.
PracticePaul Graham gets this one wrong: while ad-libbing is indeed the polar opposite of reading your slides, it's also sub-optimal. You need practice to get timings right, to try out different phrasings of your thoughts, and to make transitions smooth. Don't fall for the false dichotomy that either you ad-lib or you practice all the spontaneity out; there's a happy medium in between.
MechanicsFinally (last and least?), a brief note on mechanics. Stand where you can gesture freely and naturally; ideally (in a small room) next to the screen. Don't stand behind a podium. Don't speak sitting down. Pacing a little bit is good.
All these things mean: you need a slide remote. Even if you are right next to your laptop, reaching down to hit the spacebar or arrow key is distracting. But if you are doing it right you are probably not right next to your laptop. The remote included with Macs is unfortunately not enough, since it relies on infrared line-of-sight. If the conference doesn't provide one, borrow one from another presenter. If you speak frequently, it's worth the approximately $40 cost to get your own so you don't have to wrestle with unfamiliar hardware when you go live.