Other than wearing tanking gear, being blood spec, and in blood presence .... it's using your abilities correctly.
DK's don't use shields, and they don't block. They'll get hit harder than warriors and pallies, but that's part of their tanking style.
DK tanking, especially surviving, uses Death Strike alot, which is the basis for Blood Spec mastery (Blood Shield). Extra self healing can be gained from Rune Tap. Bone Shield should be up as often as possible to help lessen dmg too.
Anything else takes getting used to and learning more/do better with.
Edited, Aug 22nd 2012 10:34am by Azaza