Don't sit on the end of the row, or leaving a lot of empty seats between you and the next person. Shift along to make space for others, then we can all sit down and enjoy being able to see/hear the talk in comfort. Too often I see people sitting on steps or standing at the back when there are empty seats hemmed in by people who didn't think before they sat.
Use The Breakout Areas
I regularly see people in talks, including my own, who are glued to their laptops, typing away furiously and laughing when someone tells a joke on IRC. If you have no intention of listening to a talk, go hang out somewhere else so the rest of us can hear the session without listening to you type. Conversations during talks are totally unacceptable, I can see no reason short of a local emergency that would make it reasonable for any discussion at all during a session - it happens a lot, please don't be one of *those* people.
Keep Yourself and Your Entourage Quiet
Following on from not talking during sessions, is to keep everything else quiet too. This means your phone(s), laptop, camera, and whatever else you have that could make noise. I don't want to hear it, so make sure it is off/silent.
Play along when the speaker asks for your input. If you are asked to raise your hand, then do so - people who are too cool aren't adding much to the occasion, in my opinion. Also please ask questions of the speaker if there is an opportunity to do so. As a speaker, having an audience of a few hundred people that apparently were so uninterested they can't think of a single thing to say or ask is kind of disappointing. To then spend the next few minutes dealing with a queue of 10% of the audience all with interesting and intelligent questions that would have been great for everyone to hear is infuriating. So please, interact.
Keep an Eye on the Time
There is nothing more annoying (this could just be me) than an event that doesn't run to time, but if the attendees (or indeed the presenters!) are all milling around in the hallway after the session should have started, that really doesn't help. Keep your schedule handy and make sure you are seated and settled before the event is due to start. If you miss the start time of a session, then either creep in the back really quietly, or just miss it. People who disrupt everyone else and miss the speaker's introduction are just damaging the experience of those who were able to get there in time.
Thanks, Good Citizens
Finally can I say a thanks to the majority of people I have met at events around the world who have been a joy to be part of the crowd with! It doesn't take much and it makes the whole experience better for us all - thanks to all of you and I hope we meet again soon :)