You request them from the WH Press Office. Granting them is at their discretion. The big MSMs are generally locked in automatically due to their "prominence" / audience size. Little outfits have it tough for the Sarah briefings. The one exception for little guys is say POTUS is awarding some medals to the Coast Guard. For that, the photog for the Coast Guard Monthly Newsletter will be given one. But, no chance for him to get one for a daily briefing.