In days of yore, when a wedding was immediately preceded by the kidnapping of the bride, the best man was the groom's brute squad -- his strongest, fastest, buddy to help him with the abduction. His counterpart was the maid of honor, who acted as a sort of goalie to protect the bride.
Today, a best man plays an important ceremonial role in the wedding party. He is usually the groom's best friend, and is head of the groomsmen. His duties include:
The best man has the groom's back. His role is decidedly that of a caretaker; he assists the groom with personal details, keeps him as sane as possible, and delivers him into the arms of his loving bride. As a best man, you may choose to have a little fun with all this.
The bachelor party can be an occasion for debauchery, but you must keep the groom and his reputation from serious damage. (Stripper: good idea. Prostitute: bad idea. Talk to the company providing the stripper about what she is expected to do.) If you feel tempted to make this night a little too memorable, just keep in mind that tradition offers the best man as a substitute groom should the original man be unavailable -- and neither a postponed wedding nor a last-minute substitution will make people happy.
It's also traditional to sabotage the getaway car in some way, but take care not to ruin the happy couple's evening. A string of cans tied to the bumper is traditional; filling the car with helium balloons is also a nice touch. Filling the car with shaving cream will lead only to irritated and sore mucous membranes at the beginning of a night that is, frankly, all about the mucous membranes. And you don't want to ruin that cute bridesmaid's opinion of you just as the party's winding down, do you?
Being a best man can be quite a lot of work. It is also, however, the most fun a man can have at a wedding without actually getting hitched.