Whether offering a toast or presenting a formal rehearsal dinner speech, prepare beforehand, practice what you are saying, and keep the material appropriate for the audience. Avoid long speeches, and don't be afraid to show emotion.
Begin the speech with an introduction. Let the audience know how you know the bride and groom. Follow with a brief, funny story about the couple that is suitable for the listeners. Express gratitude to the couple for the opportunity to speak, and end with a memorable quote. Plan the transitions from one topic to the next so they go smoothly.
Unless you are an experienced speaker, make notes for the speech. Prepare them several days beforehand, and write them on an index card. Make sure the notes are legible so they provide the reminder you need.
Practice the speech once you write it. Give it in front of a mirror, and practice speaking slowly and clearly so the audience can understand each word.
Sometimes the things said stir emotion. As the speaker, don't try to hide your emotions. People who repress their feelings as they speak come across as robotic. When giving the speech, remember to have fun. Speakers who enjoy themselves are more enjoyable for their listeners.