It should go without saying that an organization should get its spokespeople media trained, according to Robert Taylor in his book Media Interview Techniques: A Complete Guide to Media Training. Journalists conduct interviews day in day out, so if your spokespeople have never had any training or practice they’ll immediately be at a big disadvantage, just as someone who has never learned to cook will probably make a hash of preparing a meal for a dinner party.
The book goes on to provide advice on a number of issues relating to media interviews, including:
Organisations which fail to make positive use of the media increasingly find themselves either treated in a hostile manner or ignored altogether. Silence is frequently interpreted as meaning that the organisation has something to hide. It is therefore increasingly important for organisations of all types and sizes that they have people prepared to speak up on their behalf when an occasion arises.
It seems to me that the author’s advice on the subject is as good as any that you are likely to get. The book gives you all the information you need to know; nevertheless, information alone is not sufficient to give you the skills which come only through extensive practice.