You need to stand up straight in order to give an impression that you are confident. This posture will lead to a stronger breath.
Say wrong things without getting frightened
In case you speak up a wrong thing then you should not worry that you made a mistake. Simply correct it or just move on. This thing can be done with practice only as it is a skill only performed by people who are good at it.
Open your mouth while speaking
You should not speak while keep your lips tight. You need to open up your mouth when you talk so words come out clearly.
During nervousness, one tends to speak fast. You should speak slowly as you will skip words and listeners would not understand what you are saying.
You should speak in a higher tone but not too loud. By doing this, you will consume more breath and automatically speak slowly.
Practice will make you perfect and will brush off your nervousness. You can record your voice in a tape recorder or even in your mobile phone. Talk for almost two to five minutes and record yourself. You should not read from any place but imagine that you are on a blind date or talking to your boss. After recording, you should listen to what you said. Figure out your flaws and work on that area. You can even practice with your friend or sibling.
You need to practice your pronunciation. In order to do this, you can get a dictionary and turn on the tape recorder. Read for almost five minutes and then listen to what you have read. Remember to focus on the pronunciation guides next to each word.