Always Be Preparing. You should begin preparing your client for deposition or trial from the minute you first meet them. Testimony, like playing the piano, can be improved by practice. While you cannot anticipate every question that they will be asked you can, and should, anticipate most of them. There is no excuse (though it happens continuously) for a client to testify they have never read their own pleadings or discovery responses. Every client witness should be given a list of potential questions and be prepared to answer them. Practice depositions are vital. Make sure your preparation of the witness is far tougher than the actual deposition or trial testimony. Also, prepare your client for all the tricks that will be played upon them and make sure they know how to answer the questions asked and not wander far afield. Give them examples of leading questions. Then give them more.
Dig, Dig, Dig: It is the rare case where a client provides all relevant information to their attorney. Either because they do not recall, or they are embarrassed, or they think it will hurt them, a client often withholds some vital information. It is important that they understand that attorneys and paralegals have fiduciary, confidential relationships with their clients and can, and should, be told everything. It always comes out. It is better that it does not come out for the first time at deposition or trial.