Public speaking can make you nervous any time. So if you have been asked to be the maid of honor or best man, then do not run. This is nothing earth shattering. You will just need to relax and then speak about your best friend who is the groom or the bride. You will just need to start and the flow will come on it own. If you speak from the heart, then holding on to the audience will not be too difficult.
Just take some time to figure out what you want to speak. You would need to keep in mind about how much you want to reveal in front of a packed audience. You might even want to check with the groom or the bride to make sure that you do not annoy them. Before you actually speak, you need to sit down and write the speech so that you are sure that you have included all the points. You must also practice it in front of people more than once if you want to be doubly sure that you do not stutter on the D day. Keep the speech short and make sure that you put in some emotions.