in sitting meditationuse blanket, keep knees warm
if you have a shaved head, use a beanie, blanket, cloth, etc. to stay warm.
protect the neck too especially in windy conditions. use scarf, blanket, etc. as necessary
be careful of wind! it can dissipate body heat and qi very quickly. if you get a sore neck, headache, don't open your windows all the way, or sit in a place that doesn't get such direct draft.
only windbreaking material blocks windif you like to meditate outside, you really need to understand this principle.
Even if you're bundled up with the warmest, thickest wool, if you don't have an outer layer with windbreaking material, cold wind cuts right through wool, cotton, fleece, it feels as if you were naked.