Whether as an attendee, a speaker, or a vendor I wanted to know what the unspoken rules of etiquette are at software conferences. Other than the blindingly obvious ones (like don't assault the winner of the iPad raffle because you didn't win).

What are some of the rules that should be followed, even if you feel they don't need to be said? i.e. what t-shirts are acceptable (like from competing technologies or conferences), playing Wii at vendor booths at Microsoft sponsored conferences, cancelling sessions due to low (one hand) attendance, showering, 'Attendees of Size' spilling over into your seat, talking and eating during sessions or keynotes, etc.

Please, one rule per answer, with the summary in bold leading the answer. Post multiple answers if you have multiple rules.