Tip #1 Wear jerseys.
Make sure you choose someone who will be on the team for a very long time (or at least the duration of your fandom). You don't want some weird mid-season, five-team trade leaving you with the last name of your archrival's second baseman on your back.
Don't know who your team's archrival is? Don't worry! Now that you're wearing all of your official team apparel, you will be subjected to their taunts and jeers. You know that guy who just spit in your nachos and gave you the finger for seemingly no reason? Find out who his favorite team is that's your archrival.
Bonus Tip: Check your local thrift store for some vintage team apparel. Wearing a really old shirt or jersey will make it look like you've been a fan forever. Pit stains suggest dedication.
Tip #2 Wear hats.
Hats are so important to jumping on the bandwagon that they deserve their own category. There are many different types and styles of hats, you've got the fitted hat, stretch fit hat, snap back hat, game hat, sideline hat, spring training hat, and throwback hat, just to name a few. You're going to want to buy them all. If you want to be on the bandwagon, hats are no longer just for bad hair days.
Bonus Tip: Wear your hat backwards so all of the losers left in the dust will be aware of the team you are now currently supporting.
Tip #3 Buy as much assorted team crap as possible.
How is anyone supposed to know what team you like if you do not have their logo emblazoned all over your crap? Let's face it, you're dumb and buy a lot of crap so you might as well make sure it has your new team's name on it. It doesn't matter if it's something that no one will probably ever see, and it definitely doesn't matter if it's something you'll ever use get the team version.
Just imagine the look on a robber's face when he goes to steal your Philadelphia Phillies mouse pad, or when he sees you're packing some serious heat a 9 mm handgun signed by LeBron James, that is.
Bonus Tip: Go with a matching team bed set for your room, but only if creating the illusion you're a fan is more important than having sex.
Tip #4 Don't watch the games.
The actual game can be incredibly boring and dull and besides, only a real fan will watch the entire game. Don't feel any real pressure to tune-in as long as you're prepared to watch the game's highlights or read about it in the paper. If watching highlights and reading the paper are too much, just ask the first person you see in the morning what they thought of the game. Remember this answer and repeat it to anyone who accuses you of not watching the game.
Bonus Tip: If you ever find yourself in a situation where you are surrounded by real fans and have to watch the game, prove how much of a fan you are by yelling loudly at the coach, screaming loudly at officials, and cheering loudly at big plays. Basically, just do everything you hear the other fans do. But louder, to convince anyone who may be suspicious of your true fandom.
Tip #5 Say these things
Before a game: "Big game tonight!" or "We've got this one in the bag"
After a win: "We looked great out there last night"
After a loss: "That ref stole that one from us, he must be from (opposing team's city)" or "Fire (coach's/manager's name)!"
Miscellaneous: "(opposing team's name) sucks!", "(opposing team's city) sucks!," "(opposing team's player) sucks!", and "I read an article on that."
If someone calls you out for jumping on the bandwagon: "My Dad's always been a fan of the team."
Bonus Tip: The most important thing is that you imply that you're part of the team. Use "we" and "us" liberally.
Tip # 6 Utilize social networking technology
Now that you are on the bandwagon, don't forget to constantly update your Facebook status about things that are currently going on with your new favorite team. If updating your own status is too much work, just like other people's statuses about the the team.
Bonus Tip: Use a picture of the team's most lovable player as your Facebook picture. Better yet, Photoshop yourself into the picture.