To maintain credibility, it is critical to remember the expert's role. The expert is not in court as an advocate; rather, the expert is present to teach or inform and/or to express an objective opinion based on solid science and the facts, never overstepping his role or overstating the conclusion.
It is critical that the expert answer each question directly, succinctly, and in terms that are comprehensible to the audience: the fact finder (usually the jurors). That means technical terms should be translated into layman's language. Where concessions are appropriate, the expert should make them.