When she flows off the bed and languidly puddles on the couch in contentment, curled against the open window you say you need to breathe at night (where in reality you like the way she curls against you when she's cold and have no problem nudging things in that particular direction) and watching a late movie, bathed in the television's flickering blue light, remember to touch her shoulder on the way to the kitchen for a glass of water.
Tousle her hair, knowing she'll come to bed eventually, cold and flowing. When she finally felines her way towards you, slipping from shadow to shadow and trying to wake you up as quietly as possible so you'll know she's there (movie over, Tokyo destroyed, again) be careful how you hold her - too tight and she'll evaporate, too loose and she'll freeze.
Cherish her viscosity - the next lung-full of moist winter air you inhale might be her.