If your body is declaring that you are not sincere in what you are saying then your credibility decreases and there is no way your message will have the impact it should have. So everything that implies relaxed, enthusiastic confidence and sincerity is vital now.
Think about the tone of your message. Is it relaxed, conversational? Then make your body language relaxed. Is it passionate, strong and powerful, then create body language that conveys that power. Is it alert and enthusiastic, then your body language will be upright and reflecting that enthusiasm.