These rules are mostly meant for the non-fursuiters who will be interacting with fursuiters, but it doesn't hurt for everyone to be aware of them. Some of these have been mentioned above in other parts of the FAQ, but they bear repeating, as they are often violated and broken in the worst ways possible at times.
For an extended set of etiquette rules, there is Santa Fox's Fursuit Etiquette for Non-Fursuiters.
Thanks to the entire channel of #fursuit on the IRC for their help in getting this section started.
o While in the fursuit lounge at a con, do not take pics of headless suiters. Better yet, do not take pics in there at all, unless you can actually get a clear shot of someone in suit, without a background full of headless suiters. In either case, it's best not to take pics in the headless lounge.
o Wash the suit. Wash the fursuiter after wearing the suit and BEFORE going out in public again. (cross reference with care and cleaning)
o Ask before taking a video or pic of the suiter, unless it's a fursuit parade or similar.
o Do not tug, pull, or maul a fursuiter.
o You may hug, but keep them hands above the waist.
o Do not go headless in public places. Stay in character while in public.
However, according to Marcwolf, this is true;
I don't think that deheading is such a big deal. True that it is one of the 'no-no's but its also very dependant on the circumstances, and the people you are around too. There will be times when you will need to interact with the organizers etc as a person and wearing the head would be seen as impolite, on the other hand popping the top when surrounded by kids would not be a good idea (chuckles).