Check with your college for the guidelines regarding the requirements of your thesis. These guidelines should include what is expected during the defense, and it is likely that each school differs slightly.
Make sure each committee member has received a final copy of your work at least a few days before the defense.
Have copies of any forms the committee needs to sign in order to pass off your thesis with you at the defense. Be sure to get each committee member's signature!
Talk to others who have already been through it. Ask them what kinds of questions they were given and prepare answers to those questions. While there's no way to know for sure what your committee will say, anticipating possible questions and preparing answers will help you feel more confident.
Prepare a speech or short presentation about your thesis. You should discuss the main themes or points of your thesis. Highlight the strengths of your work and explain why it is important and relevant to your field.
Be passionate. If the committee members see how much you care about your subject they will be engaged and more likely to believe in what you're saying.
Be yourself. Let your personality shine through in your work. If you're not comfortable with yourself and your work, you may appear insincere and even incompetent.