If the flask itself isn't a thermos-type one, I'd say get the water hotter than it has to be before you put it in the flask. Then wrap the flask in a "cozy" of some sort. If you have nothing else, try using a flannel or quilted dish towel. Ordinary dish towels will work, too, although probably not quite as well.
By
02/12/2010
Fill the flask with the hottest water you can to preheat it. Then fill with the boiling water you are trying to keep hot. This may sound redundant, but preheating the flask makes it keep things hot longer. Save the preheating water for preheating again, or use to water plants when cool.