We've put together a list of duties that you, TheBestMan, are responsible for. Now remember, since you're not the one getting married, this endeavor is a piece of cake. In fact this wedding is like a jelly-filled doughnut, and you are the entire Los Angeles Police Department. You're all over it! Hell, even the Groom may not know what to do, (or what he's in for) but you will, and he should be able to count on you.
1) Witness the signing of the marriage certificate. In most states, the signing of the marriage certificates, by law, needs to be witnessed before a couple can be legally married. You should offer to step up and do this for the happy couple. They may not need you for this task but at least you offered. What a guy!
2) Organize the groom and groomsmen in picking up the Tuxedos. Weddings are very big on symmetry. If something is wrong or missing, it will stand out badly once you reach the altar.
Pay close attention to this one because invariably someone forgets their cummerbund, or cufflinks, or shoes. You should be there when they pick up their stuff so you can hold their hands for them. Remember, they're groomsmen and therefore only in this for the beer. They can't be held responsible for anything. In fact it's a good idea to bring some extra shoes, socks, etc., just in case.
3) Pick up out of town guests from the airport and arrange for them to get to the wedding.
Sounds like fun, doesn't it? Again, you probably wont have to do this but you are a prince to offer. The nicer you are to the bride on her wedding day, the more fishing trips and poker nights the groom will be granted with you.
4) Make sure the groomsmen know how to escort the wedding guests to their seats.
This is always an awkward moment; If a couple arrives together, you introduce yourself and shake hands with the male guest. He should then introduce his female guest to you. He may not know this drill so just let it drop. Possibly, you will have the chance later to introduce yourself to her properly. Say, at the bar.
Ask them if they wish to sit on the bride's side (left facing toward the front) or the groom's side (right facing toward the front).
Extend your right elbow to the female guest and walk her slowly down the aisle, with the guy following you and his chick. It's a weird convention, but go with it.
If a single female arrives, you must rush to her aid at once. You wouldn't want
Oh yeah, if it's a guy that needs a seat, you can just keep it simple and tell him to sit where ever he wants, I mean, who cares. But don't let him sit next to the pretty babies, they are all there for you! Don't forget that.
Also, the groomsmen should know if there are special instructions for the seating of extended family, like cousins, uncles, aunts, etc.
5) Help the groom get dressed before the ceremony.
We know, we know, this sounds like an unnecessary step, but he may need some moral support, an encouraging word, or a shot of tequila. This is also a great time for you guys to bond one last time before he's gone for good. Maybe you could take this time to fill up your flask with the rest of the tequila.
6) Talk to the clergy/officiant and find out if everything is in order.
If it's not, put it in order. You may need to go over the names of parents, or flower girls, or ring bearers, or maybe just talk to him to make sure he's not drunk. Tell him a joke. No, not the stewardess one... something clean.
7) Hold the Ring and present it during the ceremony.
Don't lose that puppy by putting it in the pocket of your rented tux (which often has a hole). It's a good idea to bring an extra ring if you can, that fits snugly on your finger. That way if you do manage to do the unthinkable and loose the ring you can casually slip yours off and hand it to the groom. The happy couple will kill you later, but none of the guests will be the wiser.
If you think the crowd is right, and you want a laugh, search your pockets frantically when asked for the ring. We've seen this go even a step further, where, as the horrified bride watches, the best man asks if anyone in the audience has a spare, the ring has been planted on a relative or friend who brings it up.
8) Present the clergy/officiant with payment.
This is not always necessary but be prepared by bringing a blank check with you just in case this is overlooked by the bride and groom.
9) Propose a Toast at the reception.
10) Decorate the getaway car.