What are some over-wintering tips for extremely cold climates? (Answer from a Vermont native currently keeping bees in Alaska)
Wind may be one of the biggest issues you face. Nothing chills a hive down more than having -40 degree air blown into every crack and opening. In Vermont, we used those black corrugated hive wraps and they were life-savers for the bees. If you can't find those, black roofing paper, stapled on the hive like a big black present also works great, and breathes well so the hives don't get too moist on the inside, which can lead to water condensing on the cover and dripping onto the bees.
Proper ventilation is key and having a way for water vapor to escape from the hive is very important. Big plastic hive covers are large enough to allow some air circulation, although we always cut upper entrances in them for ventilation and for the bees to get out if they were in deep snow. In Fairbanks, they over-winter indoors because it gets down to 60 or 70 below, but outside of Anchorage, where I live, we rarely see 45 below for more than a few days in a row and the roofing paper is the insulation of choice, mostly because it is readily available. The black helps the hives heat up a little when the sun does come up again.
The other thing I do for cold is I move all my hives close together at the end of the season, preferably on the lee side of a building to block the wind. When they are all lined out in 2 rows, back to back, they can share heat, not unlike apartments share the heat from the neighbor's wall.
Another problem, as I'm sure you know, is getting enough food packed in there while the bees still have time to take it up. In Vermont, we made sure we had at least 100 pounds of feed in the hive to get through until we could check them in April. In Alaska, it's closer to 150 pounds. I rarely lose bees to cold directly, I lose them to starvation in the spring because they had to consume more calories to keep warm than I provided them with. As soon as you've got your honey off for the season, go to Costco and buy enough sugar to start feeding until they are pretty packed. Bring water to a boil and dump as much sugar as will go into solution. You can't make the stuff too thick. Start feeding as early as you can because once it gets cold and they're in their cluster, they are not going to be able to use food that is in the feeder, they must have everything stored in the cells. I've heard of people sprinkling granulated sugar on the top bars in a pinch. Never tried it, probably can't hurt, not sure if it helps, though. I don't rely on it. And all that honey stored in the frames acts as insulation as it absorbs the heat the bees are generating. Empty combs don't insulate so well.
Get the bees condensed into 2 or 2 1/2 supers. Stacked higher than that, they have too hard a time keeping the space warm once their numbers have dwindled. If you're treating for disease or mites, do so during the earlier feedings, so you go into winter with as few parasites or ailments as possible.
If in the past you've found that your bees have had dysentery during the winter, it may be the fault of the honey they are over-wintering on. In Vermont, the aster honey we got in the fall made them quite prone to that during the winter. We'd extract all of it and replace that with sugar and they did much better. In Alaska, we don't have the same problem with the aster, perhaps because we have a lot less of it. I hope this gives you some ideas for how to get your hives through the winter.