I come to you with a rather macabre scenario, this is something I indulge in quite regularly.
Put on makeup. First and most important, if there is one thing you must do is be presentable. Think of the doctors, the family members, the photos...
If you have time, change. Something warm and comfortable. Much like what you would wear on a long haul trip: a pashmina perhaps, a parka in case it's cold, nothing too fancy and definitely no heels because nobody will take you seriously and they will let you die.
Text your loved ones. Do not say goodbye, but do casually throw in the phone number of your best friend/boyfriend and nearest hospital if they don't already have it
Pack a bag with the essentials. Phone charger, kindle (in case the death extends like one of those horrible 24 hour labours), a banana for a potassium hit...