I've personally cut quite a few of these. They cropped up on Nikita often as well. My opinion is to break it down into it's main food groups:
- normal BG (the BG without the press conference - the roomtones or airs, birds traffic if outside, etc)
- walla bed
- walla callouts
- camz bed
- camz onscreen
- movement/knockabout
The walla will have to be shaped for the scene. Usually you have a busy/active crowd (bordering on yelling, but not riot - depends on the visual energy of the scene - run and gun vs an organized press conference). You want to get out of the active walla when the subject enters frame and is ready to speak, usually waiting for the crowd to die down - this is your cue. As the crowd is settling, cross into murmur/whisper type tracks - not a lot but enough to keep the crowd "present". As the crowd is dying down, an occasional cough or throat clear and be a nice added touch (walla callouts). As the subject is finishing speaking, I like to rise the energy back in quickly to get that aggressive type feel that the subject is being drowned out and pounced upon. Finding the right crowd source material is key though, and blending is key.
With cameras, I like to do a spotted mono track as filler to play throughout (but gettng sparse during the "press moment". Then the onscreen cam flashes will be cut for nice interplay and overall fill.
Under that, knockabout movement helps especially for the murmur section. Again dpends upon where it's happening. If a white house type room, some nice chair movement tracks are useful. if it's an ENG press conference, having some light FS shuffles and camera gear movement works. Again, like the murmurs, something to keep the press alive.
Rinse and repeat the above if you're doing a press conference futz POV - just mono prints (versus stereo) that get grouped together for the mixer to know they should all get futzed.
Hope that helps!
Also, the White House press conference in Apollo 13 (where they have the small rocket models on the table) is a VERY FANTASTIC example of a well-done press conference workup, as is the one in Contact - the one in Contact is a great example of what do with those quiet moments and how you keep it alive. The inquiry debrief near the end is also a great example of these quiet moments in a press conference.