You should always do this, it sharpens your mind. If you have a friend who will read enough of the case to act as the witness, so much the better. You can do it by talking to yourself of course- just saying the words helps you later on when you are doing it for real.
But there is another way - choose a witness who is of little importance from the prosecution list - go and talk to that person. Use it as a rehearsal so that you can learn the kind of reaction you get and how you can overcome it. If you fail, it may be a slight loss - but it is better than making a great mistake with an important witness.