Giving a press conference is much different than giving a normal speech. One must be prepared to both articulate whatever point he is trying to get across as well as be ready to answer any questions that may be asked.
Begin by knowing about the space where the press conference will be located. Find out if the press conference will be held indoors or outdoors. This could affect how loud your voice needs to be during the press conference. Know whether or not there will be a podium for you to stand behind. Making yourself aware of your surroundings will make your more relaxed once the time comes.
Consider the situation when thinking about your tone of voice. There are press conferences for practically any type of event, whether it is a somber moment or a big announcement. How you sound, mainly your tone of voice, should reflect the overall mood of the press conference. Remember that you are literally the face of the company or organization during this event. Whether it is stoic, relaxed or somber, your body posture and tone of voice will be remembered and scrutinized by those covering the press conference.
Plan how you will stand during the press conference and then practice that technique. This is another reason it's important to know your surroundings, such as whether there is a podium or not. If there is, plan where you will place your hands and then practice keeping them there. Moving your hands from position to position or constantly shifting your feet brings attention away from the words you are saying. These things also show others that you are nervous, something you probably don't want to convey.
Know the message you will be sending before the press conference and work on delivering that message. This doesn't necessarily mean knowing what you're going to say, word for word. Make yourself aware of what it is you want others taking away from this press conference. This will help you to stay on topic during the question portion of the press conference and keep you confident while speaking and responding to those questions.
Plan for any and all questions during the press conference. The best way to do this is to practice with a group of people. Have these people come up with questions before a practice session. Make sure they don't tell you the questions beforehand. You need to be ready to face “out of the blue” questions during the actual press conference. Go through at least two practice sessions before the actual press conference to both work on your answers and get you more comfortable.