I often get asked what my rotation is, and the idea of "rotations" as something to strictly follow is a pretty strong one in the WoW theorycrafting community, but personally I don't feel they're as valid for Death Knights as a spell priority. There is a particular sequence I always use for starting combat, but once that has been established it's largely a matter of using the best abilities for the resources I have available. Death Knights have a complicated resource system, and it lends itself to a much more reactive playstyle than, say, a mage. As always, my advice is heavily biased toward Frost, but can be adjusted for your spec of choice.
I should start by saying that I can't lavish enough praise on Glyph of Disease. It's vastly preferable to spend one Blood Rune that'll flip into a Death Rune to refresh your diseases on all targets than it is to spend a Frost/Unholy pair that could be spent on an Obliterate or a Howling Blast. I'm also fond of Epidemic from the Unholy tree for letting you refresh them less often. Disease management is important, but anything you can do to keep it from taking too many resources from other things is worth doing.
A good Death Knight will remember that their starting rotation is highly situational. If you're tanking multiple mobs, you want to start with Death and Decay, then Icy Touch > Plague Strike > Pestilence. From there it's just a matter of hitting Howling Blast whenever possible, especially if Killing Machine or Freezing Fog proc, Rune Strike when it becomes available, Frost Strike when you have the spare Runic Power, Obliterate if you have the runes for it but Howling Blast is still on cooldown, Pestilence whenever your diseases need to be refreshed if you glyphed for it, Icy Touch > Plague Strike >Pestilence if you didn't, and Blood Strike whenever you need to flip an extra Blood Rune.
When tanking a single target I personally skip over D&D and go straight to Icy Touch > Plague Strike > Obliterate, then Blood Strike/Blood Tap to churn out some Death Runes, Rune Strike/Frost Strike because I have some RP now, and then I move to a minor variation on the priority above. A good single-target priority is pretty much the same as the AoE priority with Obliterate replacing Howling Blast. If Freezing Fog procs, use it. Try to fit it in when your runes are on cooldown and you don't have enough Runic Power for a Frost Strike, since it's not as important to get maximum AoE out on a single target, but don't let it go to waste because even on a single target it's a nice chunk of damage and damage, of course = threat. If you have Glyph of Disease it's best to just hit Pestilence to refresh your diseases when needed, since even on a single target it's cheaper than Icy Touch > Plague Strike again and you get a Death Rune.
I'll note that I don't bother with Blood Boil much because Death and Decay generally provides all the AoE aggro I need until Howling Blast is available, and since it doesn't refresh into a Death Rune it's not as good a use of a Blood Rune as an ability that does.
There's also the matter of your mitigation cooldowns. Those are too situational to hammer into an iron-clad rotation, and on top of that hitting them takes away resources that would throw your rotation off if you stuck to a set one, so the fact that you need them pushes DKs further into a "priority" playstyle rather than a "rotation" one. I'm personally fond of tanking trinkets with a Use effect, as that gives me a mitigation cooldown to add to the rest that doesn't use any resources. I wouldn't turn down a significant upgrade in favor of one, but I would probably hem and haw over it for a bit first.
Tomorrow I'll be back with a look at optimal Frost DPS priorities. Spoiler alert! They're very similar to the tanking ones. :)