compliment the bride - she looks pretty
compliment the bridesmaids - they look pretty (unless they dont) and/or how much they helped the bride
a few jokes but none at the expense of the couple. its not a 21st speach. some gentle ribbing of the groom as long as it is really complimentary ribbing (my best man made a joke about how i was a nerd at university but really turned it into how im a hardworker story)
compliment the groom - how good a friend he has been etc
compliment the couple - how good they are together
propose a couple of toasts (to the bride and too the couple)
if you can, back the compliments up with some amusing anecdotes.
its a simple formula but ive seen it go wrong (oh so wrong) on so many occasions.
its a big honour to be asked to be the best man and obviously the groom thinks highly off the best man so it is always very nice to hear complimentary things from him. guys dont usually compliment each other.
one wedding i went to the best man did a 15 minute speach consisting largely of stories of how hed broken the hearts of loads of girls before he settled down and about all his (womanising) antics. it was one of the most embarrasing things ive had to witness.