When there was a particularly useful or interesting figure on a slide, I took a photo of it, and added "[photo]" to my notes at that point. Then, when I got home, I downloaded the photos, straightened them up, cropped them, adjusted the contrast, and pasted them into Evernote in the right places. That worked even better than scribbling down a diagram. But it did mean that I had to remember to sit near the front.
So now I have enough trials to be convinced Evernote is the right approach, for SF conventions, for seminars, and for conferences. Legible, searchable notes are a boon.