Remember, you can never be over prepared for court. Beyond reviewing the specifics with the attorney, clients have several other vital responsibilities.
Presentation: always dress comfortably, but not at the expense of a proper and respectable appearance.
Respect: Be on time, don't curse, address the judge properly, don't raise voice, etc.
Relax: Even though the experience can be stressful, it is very helpful if you are relaxed. Listen to all questions asked in full. Never think of an answer before the question is finished. The opposing counsel will pick up on this and use it to their advantage.
Pronunciation: Be as clear as possible. If you are soft spoken, you might have to talk more loudly. Whatever is said in a court room should be heard by everyone. If an answer is not clear, someone will ask for it to be repeated. It is rarely to the benefit of the person on the stand to have to repeat anything. Be as direct as possible. Speak as normally do. For some reason people feel they must use formal language in the court room, when it only comes across as a falsehood.
Honesty: It is essential that all witnesses tell the truth. If your case can destroyed by the truth, you should not be in court in the first place. An out-of-court settlement would be a better choice. Dishonesty in the court room is obviously looked down upon, and will only worsen any situation.