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.