I wrote a guide a while back on my blog for DK tanking. You might find it useful, as a lot of this was figured out while 5 boxing shamans + DK and a mixed group + DK.

Personally, Frost is probably going to be your best bet for tanking heroics. Especially in a group of retadins. You may want to start working on T10 2pc for the DK ASAP so you can get the increased DND damage as well, which also has proven to be a great boon for AE threat.

And I can't stress this enough... macro /cast Runstrike to every ability you use on your DK tank. If the ability is up, you need to use it.

My tanking guide: http://srsbusiness.org/?page_id=235