The theory is as follows. If you can get even three small things out of a talk, it is a successful talk. And if you can’t get even three small things out of a talk, it was not a successful experience. Note that the things you get out of a talk needn’t be the things that your neighbor got out of a talk, or the things the speaker expected you to get out of the talk.
Here is how it works. Take a clean sheet of paper, or an index card. Your goal is to have three things, and only three things, on this sheet at the end of the talk.
My own practice is that at the end of every seminar talk I should have a question to ask the speaker. Not that I always, or even usually ask! But I find “What more do I want to know?” to be a good mantra for maintaining engagement with the lecture.
Share your own best practices in comments!