Holding a press conference is usually only necessary to make a major announcement. A representative or two from your company speaks, making the announcement and elucidating upon it, and the conference is open to question and discussion with the members of the media. The speaker should be thoroughly prepared for all possible questions. In general, the speaker should be someone who is well spoken, charming and able to address negative matters in a positive fashion.
As a general rule, never hold a press conference if a press release or a few telephone calls will serve your purpose.