One thing people don't tell you about tackling big meetings, such as the Society for Neuroscience where I'm currently at, is that one has to perfect the art of looking without looking. That is, when you have a list of about 20 posters you might want to visit in the next couple of hours and have to decide which ones to spend time at, chatting with the presenter. But the trick is in making this decision by giving a preliminary glance at the poster without engaging the presenter. Otherwise you might be stuck and, out of politeness, you might have to stick around and listen to the whole 20 min spiel. I usually do a super preliminary walk by and asses whether the person put effort into making the poster. A bad poster most of the time signifies bad data. It does not necessarily have to show aesthetic appeal, but rather that at least some thought was put into it. I also see whether there is too much data. My attention span at meeting is short and I cannot focus on a poster with 60 graphs on it. After the first walk-by I circle around and have another look. I usually wait until the presenter is engaged with someone else so I can casually approach the poster and peruse its results. If its a good poster I can usually get the punchline with a few glances and that usually is enough. I take a few notes and go. If the data is interesting, then I look the presenter in the eye and ask for the spiel. I can usually handle about 6-8 of these in-depth views in one poster session before my brain starts to get sluggish. The rest I give cursory glances and hope I don't get ensnared. That way also whenever I present a poster I just let people know to tell me if they'd like the spiel or have questions, that way they don't feel pressured to stay.
That being said I saw some great posters today, by many of my collaborators and some by fellow bloggers, where I showed up incognito (sort of). I ran into my postdoc at some point who is attending this meeting for the first time, and his brain looked much more fried than mine. Maybe he doesn't know about the drive-by poster watching technique.