Death Knight DPS Rotations

As a Death Knight, there are 4 key rules to DPS:

1. Always keep your two diseases up
2. Use your primary DPS strike as often as possible
3. Use your secondary DPS strike to get Death Runes, which feeds back into letting you use rule #2 more often
4. Use a runic dump once rules #2 and #3 fill out your runic power

Real-world application, on a spec-by-spec basis:

I. Blood spec

The spec: http://wotlk.wowhead.com/?talent=jfVMqh0IsbRxzAZZfMhhhck

Plague Strike -> Icy Touch -> Heart Strike 2x -> Obliterate -> Death Coil until RP is depleted -> Plague Strike -> Icy Touch -> Heart Strike 4x -> Death Coil until RP is depleted

Plague Strike and Icy Touch cast your diseases, which fulfills rule #1
Heart Strike is your primary DPS strike, which fulfills rule #2
Obliterate converts its Unholy and Frost rune cost into Death runes via Death Rune Mastery, which you then use for more Heart Strikes, which fulfills rule #3
Your runic dump is Death Coil, which fulfills rule #4

II. Frost spec

The spec: http://wotlk.wowhead.com/?talent=jfVMVZGMxtIxdRhoVosx

Plague Strike -> Icy Touch -> Blood Strike 2x -> Obliterate -> Frost Strike until RP is depleted -> Plague Strike -> Icy Touch -> Obliterate 2x -> Frost Strike until RP is depleted

Plague Strike and Icy Touch cast your diseases, which fulfills rule #1
Blood Strike converts its Blood rune cost into Death runes via Blood of the North, which you then use for more Obliterates, which fulfills rule #3
Obliterate is your primary DPS strike, which fulfills rune #2
Your runic dump is Frost Strike, which fulfills rule #4

In this case, rule #3 comes before rule #2 because Plague Strike and Icy Touch use an Unholy and Frost rune, respectively, which are the same runes needed by Obliterate. You use your secondary strike while you wait for the runes of your primary strike to come back.

If Rime is up and you are in the Frost Strike/Runic dump portion of your rotation, use a Howling Blast, since Rime makes it cost nothing

III. Unholy spec

The spec: http://www.wowhead.com/?talent=jfVMVZZfMhhhckgcsMcsut

Plague Strike -> Icy Touch -> Blood Strike 2x -> Scourge Strike -> Unholy Blight -> Scourge Strike 3x -> Death Coil until RP is depleted

Plague Strike and Icy Touch cast your diseases, which fulfills rule #1
Blood Strike converts its Blood rune cost into Death runes via Reaping, which you then use for more Scourge Strikes, which fulfills rule #3
Scourge Strike is your primary DPS strike, which fulfills rule #2
Your runic dump is Death Coil, which fulfills rule #4

As with the Frost rotation, Plague Strike and Icy Touch share the same runes as your primary strike, necessitating that you use your secondary strike first while the runes spent on the disease setup gets regenerated.

IV. Dual-wield Tri-spec

The spec: http://www.wowhead.com/?talent=jfV0VZG0ethxdRhoZfM0hxco

Plague Strike -> Icy Touch -> Blood Strike -> Blood Strike -> X -> Death Coil until RP is depleted

If Rime is up and you are in the X part of the rotation, use Howling Blast, since Rime makes it cost nothing. If Rime is not up, use Obliterate

Plague Strike and Icy Touch cast your diseases, which fulfills rule #1
The tri-spec does not have any Death Rune talents, which is in violation of rule #3. However, Blood Strike is still used to consume Blood runes, which would otherwise just stay static.
Obliterate during Rime and Howling Blast otherwise is your primary DPS strike, while fulfills rule #2
Your runic dump is Death Coil, which fulfills rule #4

This is a different kind of spec, which leverages three key talents:

Blood-caked Blade, which generates extra 50% weapon strikes (with two diseases active) based on 30% of your auto-attacks
Killing Machine, which increases the crit chance of your next Howling Blast (or Icy Touch or Frost Strike) after an auto-attack crit.
Howling Blast, whose damage is independent of weapon damage, making it an ideal 'strike' for one-handed weapons.

The rest of the talents (Dark Conviction, Bladed Armor, Nerves of Cold Steel, Ravenous Dead, Necrosis and Shadow of Death) are all based around maximizing auto-atack damage.

You will want to use a slow main-hand coupled with a fast off-hand. This is because Blood-caked Blade's extra attacks will use the main-hand's damage, even if it was the off-hand that procced it.