Fair play at tournaments:
As an instructor, bow out as scorekeeper, timekeeper, or judge if your student is competing in your ring. Leave no question as to the impartiality of the results. Most instructors, although they try to be fair, either end up going harder (most of the time) or easier on their own students.
As a parent or competitor, if a judge or arbitrator rules against you, live with it, accept it, and don't make a fuss. Its just a tournament. There will be others.
As a parent or instructor, don't lie about your child or student's age or rank to place them in an easier division. Let them earn their trophy and don't cheat other children out of theirs.
As a promoter or coordinator, try not to combine brown and black belt divisions. At that rank, the competitors usually won't object, but there's a huge skill difference between brown belt rank, especially someone who JUST got their brown, and black belt rank, and most of the time the brown belts get slaughtered. (I once saw a tournament that combined six adult brown belts with one fourth degree black belt. yep, guess who won...)
Fair play in class:
Everyone is there to learn. Respect that. Don't interfere with other's learning. If you don't want to be there, leave or sit down. Don't interfere with class because of a bad attitude.
If you're fighting someone who isn't as good at it as you... don't beat them up. Help them up. Fight them at just above their level so they can get better at it.
If someone asks you to show them something they don't know, teach it to them. With rank comes responsibility, and if you're wearing a higher rank than they are, you have knowledge to share. (depends on your studio policy, of course...where I train, high ranks are required to help out if asked. at some places, only the instructor is allowed to teach stuff)
Respect everyone in the studio, not just the black belts. Being an orange belt doesn't mean you can order the white belts around. If you brag about something, be prepared to back it up. Remember, a closed mouth gathers no feet.
Observe proper personal hygiene. BO is not an acceptable martial arts weapon.
Just my $0.02
Nightingale