Asking is important because there's no one right way to be a best man. It could be as simple as showing up reasonably dressed and be prepared to go get extra lemonade if it runs out during the reception -- a task that appears nowhere on any official list of duties that I've ever seen. That task, though, speaks to the basic function of the best man, which is to provide the groom with whatever sort of help and support he needs to get through the event.
Organize the groomsmen/ushers.
In the U.S., the best man is often the leader of a cavalcade of male attendants, all of whom have to be fitted for tuxes. The best man can take some stress off the groom by being the "point man" who sees that all of the guys get measured and fitted on time. While you're at it, get a "day of the wedding" itinerary from the groom. This will tell you when you're supposed to be ready for photos, whether you're expected to keep a grip on the groom's tips for various vendors, and other facts you and the ushers need to know.
Hold a bachelor party that won't ruin the wedding.
It's not a successful bachelor party if the bride calls off the wedding when she finds out about it. Even if the groom expects something wild (and his wishes should definitely be consulted), your job is to see that things don't get so out of hand that he isn't capable of getting married the next day. (Hint: hold wilder bachelor parties a couple days before the wedding so everyone has time to recover.) Milder alternatives to bar-hopping and strippers include paint-ball wars, a group trip to a sports event, a hiking weekend, or even a fun movie together.
Keep the groom stable on the wedding day.
Over-excited people tend to forget to eat, which means that the first glass of wedding champagne hits them like the proverbial ton of bricks. Brides often plan "emergency kits" with snacks for the hours before the wedding, but if the bride is insisting that the groom doesn't see her before the ceremony, the groom won't get any snacks! You can help by seeing that he eats a little something (a peanut butter sandwich, a banana, and some fruit juice or sports drink would be great) 60 to 90 minutes before the ceremony.
While you're planning an emergency kit, some items to include are a roll of clear strapping tape (great for repairing hems), a small scissors or pocket knife, a mini-stapler, a black permanent marker, a collapsible cup (useful for taking aspirin when your only source of water is a restroom sink or a drinking fountain), and a small pack of baby wipes (not for a baby, but because they clean up a remarkable array of stains). Cash is also a great problem solver, so bring some. Finally, if you want to be the best man who saves the day, stash in a safe place in your car a CD with the correct version of the happy couple's first dance song. There's a special version of Murphy's Law that decrees that, if the DJ forgets only one CD, it will be that one (and never the Electric Slide).
Give a toast.
The traditional time for the toast is during dessert, but the happy couple may have a different schedule, so ask about it and follow it. The best man's toast is, traditionally, to the bride. Tell her what a great guy she's marrying (and don't tell any stories that would contradict this claim!), wish them happiness, raise your glass and drink, and then sit down! It's better to give a toast that's too short than one that's too long, so keep it to two minutes or less. And be cautious about "off-color" humor. The groom may think it's funny, the bride may think it's funny, but if the groom's grandmother or the bride's boss don't think it's funny, the joke will go over with a thud. (While you're at it, forget that you ever knew the names of the groom's prior girlfriends. The more you remind yourself that he's marrying Mandy, not Cindy, the more certain you make it that you'll end your otherwise flawless toast with "Here's to Bill and Cindy!"
Be an assistant host.
You'll win points if you don't run off to talk to your friends as soon as the reception starts. Help the infirm find seats and the confused find clues. Put in a courtesy dance with the bride, the maid of honor, the mothers of the bride and the groom, and anyone else who looks lonely. Keep an eye out for guests who have had a bit too much to drink, and intercept Auntie Mame before she grabs the microphone and starts singing the Copacabana song.
Dates of best men will doubtless fix their attention on the advice to dance with many women and shriek, "But I don't know anyone there -- he's supposed to spend time with ME!" A wedding reception is not a movie or a play, where you sit with your date and watch the show. It's a social event, which means guests are supposed to socialize. Since the best man probably does know several of the other guests, it's his duty to introduce his date to some of them so that she isn't stranded with strangers while he's doing wedding stuff. A really perceptive best man (the kind who has a great future as a groom at some subsequent wedding) will think about which of his acquaintances his date might enjoy meeting, rather than just assuming that she'll love the wives of his college buddies.
Be a good sport, not a party animal.
This rule, which should be the motto of best men everywhere, covers a lot of territory. It means that, if you know you can handle six drinks, no problem, you stop at three. It means that you don't decorate the "get away" car with any substance you wouldn't want smeared on your own dream car or your best suit. (A good car decorating kit can solve that problem!) It means that you don't yell "Smash her, smash her!" when the groom feeds the bride the first bite of cake. It means that you don't use the disposable cameras on the tables to take pictures that you wouldn't show your grandma. It means that you don't, in leaping for the garter because you don't want some other guy putting it on your girlfriend (who caught the bouquet), you don't perform any manuevers that would be penalized in your favorite sport.
Most important, don't be the first to mock the ritual of getting married. Sure, in the old days, the groom's job was just to show up -- but today, it's not uncommon that the big formal wedding is at least partly the groom's idea, and that he picked some of those oh-so-mushy readings that you're hearing (along with the elephant-gray tuxes with lilac vests that you're wearing). Even if he's been hog-tied into his tux and he chalked "HE - LP" on the bottom of his shoes so the congregation would see it when he kneels, he probably doesn't really think that he's a pathetic idiot to be marrying the bride, so you don't help anything by trying, even in jest, to change his mind.
Finally, even if the bride is the best sport in the world herself, don't, in the final week before the wedding, email her the infamous "If Men Were in Charge of Weddings" page. To do so is like jumping into the trenches in the middle of a battle to lecture a soldier on pacifism. No matter how right you are, you aren't helping with the immediate problem, and you're likely to get shot by one side or the other. Save it until the bride's best friend gets married, when she'll have a whole new perspective on "every detail has to be perfect."