Every story has a beginning, middle and an end. If you conclude your speech in a way that refers back to your opening comment, you're half way to sounding professional.
Unless you're a professional actor! You should be concentrating on what you're actually saying. Use a list of bullet points on a piece of paper, rather than reading out every word. It should help convey a natural feeling in your speech, rather than sounding like a pre-recorded message!
Preferably someone who will give honest feedback! Also, its usually a good idea to keep your speech to no longer than 10 minutes, timed when you're speaking slowly, which on paper is roughly about 1,000 words.
If the big moment has arrived, and you're feeling absolutely terrified, then before you start the speech, ask the people at the back of the room if they can hear you. It's an old trick, but it allows you to get those first few words out, it will get a response from the crowd, and should help you settle.
Too many best men write their wedding speeches with a few friends over a few drinks. This can be fun at the time, but often results in a speech high on rude stories and innuendo but low on sincerity. Strike a balance. The audience will enjoy some jokes at the groom's expense, but in a context that demonstrates he's a good friend with admirable qualities. Also, a touching and sincere note is a great opportunity to end with a toast. If you're the father of the bride or best man, toast the couple. If you're a groom, toast your brand new wife.
Giving a speech about someone you love is like giving them a present you know they'll adore. It's a celebration, not a punishment!