I. Thou shalt have only one favorite team in any one league.
This means if you are asked what team you root for in the NFL, you can have only one team in reply. It is a sin to say, “I like the Cardinals and the Jets.” Sure, it is ok to like some teams more than others, or more appropriately to hate some teams less than others, but one team has to dominate all other teams; you can have only one favorite team. MLB is one league now too.
II. Thou shalt make the college you graduated from your favorite college team.
Undergraduate schools come before graduate schools (and graduate schools come before jobs if you work at a university). Bottom line: If you went to say Bowling Green, then BG is automatically your favorite team. You can root for Ohio State, if you must, but if BG plays OSU, you have to root for BG. If you didn't go to college or if you went to some DIII school or whatever, you're a free agent and can pick your favorite college team, but choose wisely: See Commandment IX.
III. Thou shalt never wear a team’s game jersey unless the team is actually playing at the time.
Wearing a game-style jersey in public outside of game time is childish. Some reasonable allowance is permitted for the time period immediately before and after game time. Regular team gear, like t-shirts, hats, etc are fine any time. Kids are exempt from this rule.
IV. Thou shalt never wear team clothing for more than one team at the same time.
This is also known as the Boston Fan Commandment. Don't wear your Patriots hat with your Red Sox t-shirt. Yes, we know, City of Champions and all that bull, but there are seven days in a week for a reason. And no, Pittsburgh fans, it doesn’t matter that the colors all match.
V. Thou shalt never wear team clothing at a live sporting event if that team is not playing.
This is also known as the Steeler Fan Commandment. Seriously, don’t be that jerk wearing a Steelers shirt and cap at a random Rockies-Padres game. There are dispensations allowed here for big events like the Super Bowl, College World Series, All-Star games, etc. where fans may wear their team gear as a general celebration of the game. Wearing a polo with a small logo is only a minor sin.
VI. Thou shalt bet for your favorite team only to win.
You don't have to bet on your team's games, but if you do, you must bet for them to win. There should NEVER be a silver-lining if your favorite team loses. Hedging is for financial markets, not sports fans.
VII. Thou shalt never bet with or against the spread on your favorite team.
If you bet on your team, bet them to win on the moneyline. You should never be in a position where your team wins but you lose a bet or your team loses but you win a bet. You can bet with the spread or whatever on other games, just not on your favorite team.
VIII. Thou shalt always root against your favorite teams' rivals.
This is also known as the SEC Fan Commandment. Don’t be that moron Florida fan (redundant, I know) who roots for Alabama in the championship game because of the conference. The only exception is that you can root for a rival if their winning helps your team immediately advance to or in the postseason.
IX. Thou shalt never change your favorite team.
This is also known as the Cleveland Browns Fan Commandment. Once a team is your favorite team, it is always your favorite team. Sorry. I know it sucks. The one exception is that if your team moves to a different city, you are free to drop that team as your favorite and select another.
X. Thou shalt have one favorite player who is playing now and one favorite player all-time.
And if you’re over 30 years old, it really should be different players.