Select a type of defense for your thesis that your evidence best supports. If your thesis supports a new theory, position or technique, explain why your thesis is better, easier or more accurate than the current standard or practice. Be prepared to explain why your thesis is able to overcome the limitations of the current standard you are arguing your thesis should replace.
Anticipate what questions you may receive from the panel and prepare a response to those questions in advance. Even if the panel doesn't ask you an exact version of a previously considered question, developing a response will give you the opportunity to practice critically evaluating your own work. Practice answering questions about your thesis with a friend or fellow classmate to get a feel for articulating your thesis' arguments aloud.
Resist becoming defensive when asked a question, and don't assume the questioner is intentionally asking a difficult or aggressive question to undermine your argument. The author of a thesis paper is usually the most informed person in the room about that topic. You may be asked to explain something because the person in the panel doesn't fully understand the topic as thoroughly as you.
Take time prior to answering a question to compose yourself and consider the question fully before answering. Students can place undue pressure on themselves to answer a question immediately because they feel any slight pause as sign of not knowing the answer. What may seem like an eternity of silence to the presenter will come across as reasonable pause for reflection to the panel.
Don't attempt to misdirect the panel by answering a question you don't have a quality response for by using highly technical terms. Such answers only serve to confuse the panel, and may result in follow-up questions that require you explain your previous answer. If you don't have an answer to a specific question, try to relate that question to a topic for which you do have an answer.
Willingly admit the limitations of your thesis if presented with a question that casts some doubt on your thesis' validity. Admitting that your thesis cannot directly account for that particular question may cause the questioner to back off, while denying the validity of his question may cause him to aggressively pursue a response. Go through the counterargument yourself and attempt to find holes in that argument and limit the damage caused in case of such an unlikely occurrence.