I use this rotation.
If its 2 mobs don't use DnD, if its 3+ use DnD.
Remember this is only a good rotation for blood tanking.
If you can't figure out the macro, the rotation is:#showtooltip
/castsequence reset=combat Icy Touch, Plague Strike, Pestilence, Death Strike, Heart Strike
/cast Rune Strike
Icy Touch, Plague Strike, Pestilence, Death Strike, Heart Strike while casting Rune Strike whenever it is up.
You may also want to consider a respec to a more effective spec.
This is my current tank spec on my DK, you can move around a few points if you want.