Speak only to what you know. Do not guess. Do not fabricate. If you do not remember, say so.
Stay calm. Breathe. It is easy to let nerves, anxiety and anxiousness to make the deposition much harder than it needs to be.
Listen attentively to the questions being asked. Most questions require a simple yes or no response. Fight the urge to say more. Even if you think a yes or no answer may not provide a complete explanation of circumstances surrounding the question or your answer. At the end of the deposition your attorney will get the opportunity to ask you questions to give you the chance to expand on your answer.