If I'm not talking in front of a big room, I prefer to whiteboard notes and diagrams as I'm talking, based on an outline that would otherwise be pretty near to what I'd put on slides.

One big advantage of whiteboarding is that you're much more likely to draw a diagram. People seem to pay more attention. If they want a take-away, you can photograph the drawings and notes.

For whatever reason, trying to do the same in a big room with an overhead projector is much harder.