Saddle up boys we are in for a good time. In the words of King Leonidas from 300, "Remember this day, men, for it will be yours for all time". Though I believe he was referring to fighting to the death against a massive army, I believe it still applies to being a groomsman!
Being a groomsman is one the highest honors in all of manhood. It means that you have created a strong enough relationship with another man that he wants you to be there on one of the most important days in his life. A day that he will never forget. The day he commits to his lifelong best friend. It means he trusts you not to ruin it, so let’s not do that! There are a few simple ways to make sure you don't.
Be selfless. Whatever he asks you to do, do your best to make it happen. If the wedding reception goes all night, you stay all night. It isn't about you. If you have to stay out till midnight tearing down after the party because your boy needs you to...then you stay out till midnight tearing down after the party. If he wants to get up the morning of the big day to eat a big breakfast, you show up and pay for him to eat as much bacon as he very well pleases. If he wants to play a sport that you don’t care for or do an exercise, you do it with glee.
Don't talk about yourself. Not all the groomsmen probably know you that well, but you all have one person in common, the groom. Focus on the Groom! Again, this isn't about you. Ask him lots of questions, remind him of stories of good times. Ask the other guys to chime in on fun stories with the groom. Ask him about his bride. Ask him about their marriage and their plans. Ask him what he is thinking about. Ask him how you can make the wedding day go as smooth as possible. Just ask him questions and have good conversations.
Unify the group. Your goal is to bring everyone together in the room. The more unified the groomsmen are, the better time the groom will have, because all of his best friends are getting along. It allows for more stories, more laughing, and less awkward silence. Just keep talking and asking questions about each other. Make connections between people. You may even make new friends for the future.
Keep it light hearted. Don't let anything stress the groom out. There is nothing that could actually ruin the day other than a pessimistic attitude (and the bride not showing up, but you have no control over that). It is your job to keep him in good spirits. Wake up early and take him out to breakfast to start the day out on a high note. Maybe go play a sport or be active so he feels fresh and fit for his wedding. Have something to do while waiting for the wedding to start so everyone is having fun. Avoid any “now that you’re married, your life is going to be boring” talk. Your friend has decided to marry this woman, do not make him second guess this decision on the day of. Pray for him and his future bride. Once the wedding has started keep the party lively. Dance your pants off! Try and get others out on the dance floor. A dancing wedding is a great wedding. Don't be embarrassed if no one else is out there. You are dancing for your friend, and eventually others will join in. For any hesitancy, revisit points 1 and 2.
Don’t get drunk. Probably one of the most important things to do is to not get drunk. If you do, then you will not be able to do most of this work because you have become selfish and put yourself in a scenario where you cannot serve your friend well anymore. You can be still be fun and enjoy yourself sober, but again, this is not about you. You can give a better speech, you can help if things need to be moved, you can drive people who may be intoxicated, you can handle any drama that arises, you will ultimately be more aware and present.
These are just suggestions and things I have personally seen work and things I have seen ruin parties if not done. Do whatever it takes to serve your friend well so they look back on this day and know that you love them and are willing to do anything for them. They will thank you forever!