I am aware that some situations call for different rules. There are lectures during which it would be appropriate to talk, eat, tweet, and/or fart. Or so I have been told. However, this is rarely the case when lectures are given in academic settings, which are the ones I have chiefly in mind here. One general rule governs everything that follows: Do not make it harder for speakers or listeners to concentrate on the lecture.
1) Go to great lengths to be sure you know precisely when and where the lecture will be given. Do not make assumptions.
2) Arrive at the right place on time, or even early.
3) It is especially important to arrive early if you think you will have to leave early. In such a situation, you will want to situate yourself as close to an exit as possible, so that you may depart quietly and unobtrusively. You might also ask yourself whether you need to leave early or would just prefer to leave early. If you would just prefer to leave early, maybe you shouldn't be there at all. Food for thought.
4) You shouldn't be late, but if you must be late, come in as quietly as possible and sit near the door. Sit on the floor if you have to. In no circumstances should you stroll in, scan the venue in a leisurely fashion, and amble towards the center of the room.
5) Turn your stupid, stupid, stupid cell phone off, and never look at it during the lecture.
6) If you plan to take notes, do so on paper. Do not haul out your laptop and make your neighbors try to listen to the speaker over the constant rattling of your keyboard.
7) Shut up. Listen to the speaker. Don't say anything to anyone at any time — unless, during Q&A time, you actually have a question you'd like to know the answer to. (Note that very few of the people who ask questions of public speakers are interested in getting actual answers.)
8) Do not eat anything. What are you, some kind of barbarian? Wait until the lecture is over and then eat in a place appropriate for eating. No one listening to a lecture wants to smell your food or hear you chew, swallow, and suck your drink. No one.
Thank you for your cooperation.