A couple of years ago I adopted a new custom. I try to write down just one thing from every talk I go to.
Everywhere I go I carry a tiny notebook, about the size of a matchbox. The one thing has to fit on one page. If I want to take more copious notes on ordinary paper I do, but I still write down one thing. I was amazed at the difference this made. It forces me to try to frame one idea from the talk in concise terms, and every now again I flick back through the notebook to remind me of old talks.
Of course it's shocking that it does make a difference, because you'd hope that on average you'd retain much more than one thing from an hour-long talk. But in my case, apparently not.
Depressingly, there are a few talks at which I can't even find one thing to write down.