At least according to me. There are certain habits that no matter what event you're at, are just common courtesy and common decency.
I will say that all have the counter-argument "I paid my money, I have a right". That is true. But being a fan is more than just exercising your rights. It's about sports, enjoying sports, and having a good time, and when you violate my 10 commandments, you infringe upon others' rights to do the same.
1) Avoid using profanity. I know it's tempting to call that overpaid SOB a s##tbag and tell him that you're not fond of his mother (or insinuate you had relations with her), particularly when he strikes out with the bases loaded or misses a tackle or misses an open jumper. Refrain. If he's one of your guys, he's one of your guys. Heckling the other guys can be part of it, but be clean and creative (think Duke fans). There are kids and families around. He can't hear you anyway, you're just venting frustration and you're better than that.
2) Only take or leave your seat during breaks in action. This is championed in hockey with the ushers and the hand-held "stop" sign, but it's not that hard to stand behind your seat in the concourse, or in case of a long aisle, squat in the aisle until it is between football plays, between batters, or a free throw. Hockey is the hardest, action can go for minutes. But it's a small price to pay, and you'll get more respect from your fellow fans rather than making them stand and missing a long pass or big 3 pointer.
3) When in Rome, do as the Romans do...when standing. Don't be the person who is the only one standing when everyone else is sitting. Don't be the person who is sitting and asking others to sit when all are standing. I know you paid for a seat and the right to sit and watch, but in college football for instance, you're not going to get everyone to sit, just go along. And if you're the only one standing, you're probably drunk.
4) If you're smack dab in the middle of a row and have to exit, choose one way out, and the other way back in. That way you don't inconvenience the fans in the row behind you twice (the standing folks can see the action while standing both times)
5) Bring cash for the concessions. That commercial where everyone swipes the card and the line moves faster is BS. I've been in a beer line with 4 people and each credit transaction takes 90 seconds costing me a half inning. Same line with people with $10's gets done in 90 seconds total. True story, I was at a game in which Bonds was going for HR 714, I left with 5 batters before he was due, they rallied and he was on deck as the guy in front of me was trying to pay, but the cashier couldn't get his card to work. I just about threw two 20's at her and ran, I'm sitting there for BS while almost missing history.
6) Mind the spillage. Keep that tall drink in your hands at all times (both hands). If you spill, instantly warn the fans in front of you to remove their personal effects from the floor. Get napkins if necessary. It sucks to have someone's drink flow over your purse or bag.
7) If you're a road fan, don't flaunt it. It's okay to cheer for your team, but turning and taunting or trying to prove some point by cheering every smallest thing is bush league. If I'm watching my team on the road and they score a TD. I stand, I clap, I make no eye contact with other fans. I DON'T say "Oh yeah, baby, that's right. D-money-jonesey, you the man, we got this baby, that's right, rah rah team". Stand, clap, high 5, sit down.
8) In step with Commandment #7, it never hurts to befriend nearby fans of the other team. Just talk X's and O's. Don't say "We're going to kick your a$$". Say "this is going to be a good game, I think we have a good team, you're team should be ready to play" or "I like your RB, I can't wait to see if he is able to run on our defense". "I heard this pitcher is good, will be a tough matchup for our hitters". I had a great experience at the BCS title game with some Alabama fans just by talking sensibly and respectfully and it made it better for everyone. In return I got "wow, Texas fans are nice, nothing like LSU or Tennessee fans". I guarantee you Texas fans can be the worst, but I just put forward our best face.
9) When you lose, you lose. Speak softly and walk away. It's just a game. Insulting the other teams star player for his paternity suit, or the other team's university for it's locale is bush league. Hey, they won, you lost. That's why you play. Silence is golden.
10) Have fun. Seems easy, but I think we've all been to a game where someone just seemed to complain about everything (it's too hot, I can't see, the concessions are too expensive, that SOB sucks, our team sucks, I can't believe tickets cost this much). Hey, you chose to be there, enjoy the game.
I'm not perfect, I've used profanity and insulted a dad and kid (Sirron remembers), I've spilled my mixed drink on the ground (okay, kid next to me kicked it) and didn't warn the people in front of me before her purse was drenched in Beamy Coke. I've been with a group who hurled insults so heavy, a nice lady in front of us tore out of the row near tears. I've been at games that by the end the whole experience was miserable, I've insulted College Station after a tough loss.
Following the above 10 isn't easy, but worth it in the end.