If you're wondering how to give the best man wedding toast, first you'll need to think like a journalist. In any good story by a reporter, you'll almost always find the 'five Ws (and one H)': who, what, where, how, why and when. Second, congratulations on being selected for this honor. You may consider it to be a chore, speaking in front of your best bud's wedding party, but you can make it as fun or loving -- or both -- as you want it to be.
If you're having trouble getting ideas together for a speech, get out some paper or your word processing program and pound out a few ideas. You can always go back and fill in the details and organize your thoughts into coherent, compelling and brilliant prose later.
Tell the audience about the two people whose wedding they're celebrating. They know them well, perhaps, and have their own point of view. But as the best man, you're expected to know the groom especially in a way that no one else does. What kind of man is he? What is your (very favorable) perception of the bride?
'We're gathered here to celebrate this joyous union between my (good, best) friend and his soon-to-be life partner.' This is a great place to start your toast. Sure, the audience already knows what you're there for, but it makes a better introduction than that corny joke you found on the Internet that you were thinking of leading off with.
The 'where' in a best man's speech doesn't literally mean the location of the reception. This is referring to details. Do you have funny stories about the groom (that you can repeat in front of your mother without blushing)? Do you recall an event where you were with both bride and groom that you think others may want to know about, or might get a chuckle out of?
How did the bride and groom meet? Tell the story from your perspective. If you introduced them to each other or if you were there when they first met, bonus! You've got a great story to tell in your best man speech.
The 'how' in this case can also refer to how you met the groom. It's a good opportunity to segue into why you think the groom is such an awesome dude that deserves such a wonderful, attractive, smart and witty bride.
Think hard on this one. Why will this couple be so great together? What is it about them as individuals that will work as a couple? How are they compatible? You can talk about how they are incompatible, but only jokingly. If you dare go this route, make sure everyone understands you are 100% kidding, even if you are 100% not kidding.
You should add color to your speech. You can quote Shakespeare, or Yeats, or other famous dead guys. Avoid quoting divisive political figures, and probably avoid quoting the Bible. If you're known for your sense of humor, load up with the quips and witticisms. If not, try and be inspirational or keep the speech full of praise, but don't step into the comedy arena if you are not well-equipped. Either way, keep the comedy light -- this is a toast, not a roast.
Don't forget the other members of the bridal party. If you don't know them all that well, say how you enjoyed meeting them and getting to know them. If you do, include praise for the parents of the bride and the groom in your speech.
Don't get sloshed before the speech. There's nothing worse than listening to a drunk ramble for 10 minutes trying to find the words to express how much he 'loves this guy.' You'll have plenty of time to drink afterwards, and after all, you'll have earned it.
Raise a glass for the toast and you're done!