A press conference is not an easy event, or a casual one. It's a big production and should only be saved for big issues. If you schedule press conferences every week, or every day, the press will stop showing up.
So when should you do a press conference?
1) When it's truly big news
A press conference is typically set only to announce huge news, something you'd want to give TV reporters advance notice and time to drive down and set up their cameras.
In politics, you'd do a press conference to announce a run for office -- or to drop out of a major race.
Same thing for other public figures. A business would hold a press conference for earth-shaking news like mergers and bankruptcies. An NFL quarterback might hold one to announce he's asking for a trade to a different team.
2) When the news breaks
The news has to be fresh. If you schedule a press conference for Friday, but leak the news to your favorite reporter on Tuesday, there's no reason for any reporters to show up to your press conference. They already got scooped.
If you're going to have a press conference, you've got to have it when the news breaks. No leaks. No favors. No hints to friendly reporters, because good reporters will sniff around and figure it out.
3) When the phone won't stop ringing
If reporters are calling you all day about an issue, wondering when you'll make the announcement, that's a good time to have a press conference.
If not a single reporter has asked about this issue, that's a sign. Don't schedule a press conference about it. Save press conferences for big news. If you have them too often, the press will stop showing up.
Caution: You'll have to talk clients out of press conference and into other events and products. "Press conference" and "press release" are the two phrases that most laypersons know. When they say "press conference," they might actually mean a media availability or a photo op. They probably don't know the menu of options; what they do know is they want to get press about something. Talk them through the options before scheduling a press conference.