First learn the meaning of what you say, and then speak.
---Epictetus
Be prepared. There is nothing like being prepared. Before writing a speech, know the purpose of your talk. Will you be toasting a couple or roasting them? How long will you be speaking? Make sure your speech reads well. Does it reflect your feelings? Does it sound like you? Can you pronounce all the words easily? If not, practice until you are comfortable with it. In addition, try to get a sense of the room and equipment before giving your speech. If you will be using a microphone or projector, make sure you know how to operate it.
Practice. It can't be stressed enough: practice makes perfect -- and it also makes you less nervous. Practice your speech standing up, as you will be when delivering it. Picture the audience in your mind as you read over the speech and see them smiling and nodding, enjoying your talk.
Relax. Take a deep breath or two. Relax your shoulder muscles. Smile. Remind yourself that the audience is friendly and made up of people who aren't likely to be professional speakers. Smile again and be yourself. Some professionals suggest imagining the audience in their underwear. I think that is plain scary, but if that works for you, imagine all you want.
Use notes. It is OK to use cue cards, but ideally they should be referred to, not read from. This is the ideal. If you need to read your speech, then by all means, read it; but pause, look up, and smile occasionally.
Speak from the heart. The best speech, whether a wedding toast or bar mitzvah speech, is a speech that comes from the heart. Sincerity is the key.
Speak clearly and slowly. Often people speak quickly when they get nervous. Counteract this by reminding yourself to speak as slowly as you can. Although it may feel like you are speaking in slow motion, it will likely sound just right to the folks in the audience.
Speak louder than you think necessary. Even with a microphone, no one can love your speech if they can't hear it.
Make eye contact with as many people as you can. People will feel you are speaking directly to them.
Smile. It will help you relax and put the audience at ease as well.
Enjoy yourself. If you are enjoying yourself, your audience will enjoy themselves, too.