This is a simple mantra that is going to change the way you attend every meeting and every conference for the rest of your life.
You probably don't have to be there. No gun held to your head, after all. So, why are you spending the time?
Do you have an agenda? It might be to change the agenda, or meet someone who will become a client or to learn something that will help you at work tomorrow.
Well, if that's why you're here, tell me again why you're just sitting there? If the only reason you came was to avoid the office, you need a new office. Quick, before the boss decides that for you.
Surely you have a question you can ask the speaker. Surely you have something interesting to say to the person sitting next to you. Surely you can do more than just sheepishly hand someone a business card they have no reason to save or remember or use.
If there isn't a good reason, go home. If there is, then do something. Loud, now and memorable. Productive too, please.