How much information should I include in my lecture slides?

Here's a general guideline:

Figure out whether you want the audience to pay attention to you, or to the slide

They can't do both. A large part of the time, you want the audience to pay attention to you and the words you're saying. At those times, the slides should be as sparse as you can make them. Really, the only function they have is as a reminder to people whose attention has momentarily lapsed: this is what I'm talking about, this is where I am in the line of the story. Also make sure, during these moments that you're not looking at the slides. The audience looks where you look, unless you make eye contact with them.

But sometimes, you want the audience to focus on something in particular. Perhaps you're taking them through steps in a proof or derivation, perhaps you're explaining a complicated diagram, you may even be reading out a quote. These are the rare moments when you're asking the audience to really focus, and raise their energy levels for a moment. Here, you're allowed to have dense slides. These are also the slides that you're spending a lot of time on, so make sure to put the effort in. Use all the visual tools at your disposal to make the message as clear as possible. At this point, you're actually allowed to look at your own slide, because you want the audience to do the same thing.

Finally, do not mistake slides for learning aids. If you want to give students something to help when they're studying, create a version with added notes and put that online. Don't clutter up your slides to make them serve two purposes.

Slides have a dual purpose: to help the lecturer through the lecture, and as a study aid for students. Because of this, I use two versions of slides: one for the students with a lot of details, and one for me with very few details. These are both produced from the same source (LaTeX), so there is minimal overhead on my part. I use my slides to guide me to tell a good story. Then the students have the whole story for studying later.

I usually adopt a method similar to Dave's. I have my course notes, which is a latex document (not a beamer presentation, to reduce overhead) and the presentation.

The presentation has very little text. You don't put text on presentations (there are arguments against even bullet points). I usually put images, diagrams, equations and etc, the things I want them to see so they can understand the underlying concepts. After, they can see all the little detail in the notes, which are on a "printer-friendly" format, since it is not formatted as a presentation...