One point besides being warm (which is a really tough task by itself), is having something to drink - it's not easy in below-freezing temperatures. You'd want to have a reliable stove or you'll be forced to share your body heat with this show filled bottle. Another option is using sun light to melt snow during the daytime: use a lens or try to heat your black sleeping mat - but this will probably work in the summer only.
Having a stove and a gas cartridge is really a big deal in winter survival. Just as a good map and a compass are.
I hope you are asking just to be prepared for some highly improbable emergency. If chances of this surviving-without-sleeping-bag situation are high, I would just cancel the trip - or get enough camping gear to make this not an emergency but a routine. And please don't go alone.
And if you get such an emergency, it's essential to think of it as an emergency, i.e. don't try to finish he planned route, don't hesitate to call a helicopter if you have the option, don't hesitate to leave some expensive but heavy/unnecessarry gear behind, etc.