I am a new tank as well; just starting tanking for the first time with a fresh 80 DK (original main was a Mage). I'd also like to see an answer to this from those who have far more experience than I.
Having said that, I am also Frost currently (2H for now, planning on DW-ing once I get the weapons to do so), and I've found that the following seems to work well: I use Unbreakable Armor liberally, since it has the shorter cooldown (1min, 50secs w/4pcT9). Icebound Fortitude I use more as my "oh snap" button, or if I know there are impending stuns. Obviously this changes if I know of a certain spot where there will be a damage spike for whatever reason; then I'll save UA as well.