1) Use "hokkairo" (chemical heating pads, one time usage). They provide 8-10 hours of heat. Stick them on your body. Buy a hot canned drink from the vending machine while you are at it, and hold it tightly and sip slowly.
2) Take a deep hot bath (at least 40 C) just before going to bed; not only will it help you sleep but you will be warmed all the way down to your feet. Dry off quickly and put on some fleecy PJs. You can prewarm your bed with a microwavable gel pack. Top a thick fleece blanket with a down quilt, and use something insulating under you if you are a futon-sleeper, like a second futon mattress, another quilt or a fuzzy elastic cornered bed cover. Bed socks are also recommended.
3) Cover drafty windows with bubble wrap. Wash the window, then press special flat-backed window wrap to the glass when the glass is still damp. Even easier, stand a specially designed styrofoam foldable draft-stopper between the curtains and the glass (both available at home centers).
4) If it snows, shovel it even if you are in an area of Japan where you know it will melt soon; even better, do as my neighbors do: sweep snow with a broom into a dustpan vigorously, as it is falling.
5) Gather around a bonfire. Our local elementary school opens up to the community (Sunday January 10th, 2016) for a bonfire which is set in the middle of the school playing field to burn last year's lucky charms and roast pink and white mochi balls on sticks; ask around if there is something similar where you live, usually in the second week of January, on the weekend.
There you have it. Keep warm folks!