Of course plan ahead and practise. Write down a speech of no more than five minutes and take a video with your phone and observe your effort very objectively and see where you could improve. Are you mumbling? This is a surefire way to be a failure on the day. How is your body language? Do you look confident or reserved and shy? Look at these things and show the video to friends for their feedback. Practise in the mirror.
Don't get wasted before the speech
You will obviously be pretty nervous and this is to be expected. Stay sober until after the speech and treat yourself to a few afterwards as a reward! Have your speech written down on paper and ready. Your opening line is paramount to your success! Show them that you are in charge. Thank the other speakers and compliment the necessary people on how successful the occasion has been so far. Mention the circumstances surrounding your first meeting with the groom and what you like and admire about him such as any sporting or academic victories he has achieved.
Keep it light
Being funny is one thing, but unless you are about to give Eddy Murphy a run for his money, you should consider toning it down a bit as more often than not, desperate jokes will be the cause of embarrassing silences or even worse, forced chuckles. This is true particularly if they are in any way insulting or silly fables from days gone by and juvenile pursuits with the groom.
No Sex- we're married!
By all means have a couple of teasers but keep them as matter-of-fact. Strictly avoid any topics which include drinking, drugs, gambling. politics, religion or sex. Private jokes that not everyone present would understand should be avoided at all costs. Mention some of the groom's idiosyncrasies of course, but don't expect that to be the mainstay of your speech or to have them all so titillated that they are falling off their chairs and have completely forgotten about the sweat dripping down your forehead.
Compliment the couple and their choices. Speak a little about the couples dating history and how you may have been involved in any way with them uniting. Why are they so good together? Have a few different stories on various themes to change the tempo a bit like Bohemian Rhapsody if you like! This will appeal to the emotions of the audience and sustain their interest.
Make a closing quip that is congratulatory and flattering. Propose a toast and kick back and enjoy the rest of the celebrations. You did it!