I'm a professioanal speechwriter...used to be anyway (I've branched out into other communications consultation).Mind you, it's hard to give advice on such a personal speech without knowing the speaker. A good rule of thumb for this particular kind of speech is to put yourself in the shoes of the OTHER person in the wedding. (if you'r the groom's best friend, put yourself in the shoes of the bride)...and use that as a filter for what's appropriate. There is nothing more poignant than the family/friend of one half of the wedding -- speaking glowingly of the other half. I've made it reccomendation number one for most wedding speeches I've helped with.
Most of the previous advice is also good stuff. Either you're funny or you're not. Timing (as opposed to material) is the best exemplar of good humour. If your not funny, don't try to be, though there are some jokes (usually self-effacing ones) that actually possess enhanced delivery from a less than funny speaker.
Without knowing your context here's a corny opening that works ridiculously well
"Thank you XXX...it's an honour to be here. Though I am a little nervous. A friend of mine told me about the time he was giving scheduled to give a wedding speech of his own. He was sitting at the head table during dessert when the groom turned to him and asked...'are you going to get up and speak now, or should we let the guests enjoy themselves a little more first....blah blah blah
Give yourself an outline that you stick too. A good wedding speech is short and reads something like this.
1) joke
2) How beautiful the ceremony was
3) how great the reception is
4) A (possibly funny anecdote) about how you came to be asked give this speech / what an honour it is.
5) One or two funny or touching anecodtes about the bride/groom (whichever one you know).
6) Something from the heart about what the person means to you.
7) Something from from the heart about their new life-partner...and how important their happiness is.
8) A nice simple toast. along the lines of "our friends don't get too emotional with each other in public, but I hope X, allows me this now. You know who you're real friends are because they're the ones whose happiness, in and of itself, makes you happy. I've never seen XX happier than he/she is today...and it fills me with joy. You are both so special to all the people here today, and we look forward to being there with you as you build this new life together...blah blah blah.
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