Don't get pregnant
I did this once, it didn't end well (the bridesmaid thing; the baby came out perfectly). So either keep your legs closed for the entire two year engagement or keep your IUD in... Either way, no one wants a baby yielding hormonal head case as a wedding attendee. Prevent it.
Know your role
If you're the slut, be the slut. Don't pretend you believe in monogamy just because your bff is getting married, she chose you because of who you are. Slut on.
Pretend you're nice
You're not nice, we all know this, but you need to pretend you are for at least 6 hours. Dance with the sketchy great uncle when he asks for your hand, eat the entree even if the food is cold because the bride made you take 13,725 photos two hours prior, smile through the horrendously long ceremony that feels more like a eulogy to your sanity, and refrain from punching the annoying bridesmaid in the throat even if she does talk about her coach purses and laser hair removal experience all night in the most ridiculous kardashian voice you've ever heard.
Be a chameleon
Learn to adapt to your scenario. Rage hard at the bachelorette party, put dirty two dollar bills into a man's sweaty banana hammock, take four more tequila shots than you intended, puke in the back seat of the cab. Then wake up the next morning and drink tea and enjoy a delicious cucumber sandwich with all of the women from the bride's church growing up. Adapt. Be supportive.
Don't get more drunk than the bride
She may need you to hold her hair back if things get too emotional, stressful, or bulimic. But really, no one wants to carry you to your room or help you find your underwear in the pool. Keep it semi-classy, you dirty slut.
No one wants to eat fish at a wedding, the groomsman smells like salami, the dress is giving you a rash... No one cares. Always agree with whatever the bride decides.
You always love the groom
This means all the time. Even if his jokes are dumb, he has no fashion sense, or he acts like a total toolbag, you love him. Unless you catch the groom cheating, stealing, or physically harming someone, just stop.
The bride is always right
Even though you need to love the groom, remember that the bride is who you're supporting. If she loves the groom, you love the groom. If the groom is being the douche you know he is and the bride is upset about it, so are you. Relieve yourself of any emotions you may own and take on the emotions of the bride. I mean, you owe her, after all.
Show up to shit
I don't know how else to say it. Be there. Don't just plan on coming to the wedding, come to all five bridal showers, make it to the dress fittings, assist with finding a venue/band/florist, whatever. Don't be useless.
Don't sleep with important people
We've established that you're slut, so it's likely that you'll sleep with someone at the wedding. That's fine, but just make sure it's neither the groom (boooo, you whore) or the bride's father. These are the two most important people in the bride's life, so don't fuck around. Everyone else is fair game.
Have a blast
You're not only there to help said bride avoid making the most terrible mistake of her life, you're also there to make sure she has fun. So make the most of it. Because your friend will only get married once. Or twice. Or three times if she's really lucky.
The most important rules for being a groomsman:
What do you think? What are your top tips for surviving wedding season?