I agree with Pyro. I grow it in a mixture of fine orchid bark and dried sphagnum with a 1-2cm layer of pure dried sphagnum on top. I water it when the top of the media looks dry.
Tamlin: if you can tell when U alpina has gone dormant, you're a better man than I. Mine has shown easily identifiable growth pattern.
I've been reading Darwin's carnivorous plant book. He examined the tubers of U. alpina (known as U. montana to him) and found that they were not used for storing anything except for water. Apparently this is unusual in the plant world, tubers usually store starch as well as water.
He also allowed it to go without water for 30 days and observed the tubers shrinking, although they became turgid again shortly after they were watered.