Use your survival cooldowns
While tank death knights get a few unique cooldowns, many of our "tanking" cooldowns are available to any death knight, including those who DPS. Knowing about these cooldowns and how they work can be just the ticket to surviving a particularly nasty fight. Use your cooldowns when you draw aggro or when you're in an AoE-heavy situation and the healer doesn't appear to be able to heal you.
Anti-Magic Shell is, unfortunately, not quite as awesome as it used to, since you need to use talent points in order to allow it generate runic power. That said, it's still certainly a valuable way to avoid some damage. Most forms of magical elemental damage can be avoided with this. If you're being focus-fired by a mage mob or if you're in the midst of a huge magical AoE (even one you were supposed to run out of), hitting Anti-Magic Shell can shed a lot of that damage and keep you alive long enough to get some healing.
Icebound Fortitude is another great damage reduction cooldown. It's only 20% damage reduction, but with huge Cataclysm health pools, that may be enough to keep you alive until the tank takes back aggro. As a bonus, Icebound Fortitude will also provide immunity to stuns, which may come in handy on mobs that do, in fact, have stuns.
Blood Presence is a very tricky thing to use as a survival method, but in a pinch, it does provide some damage reduction and extra health. However, it also provides extra threat. This means that if you're retreating into Blood Presence to soak up some extra damage, you should also stop DPSing until the tank peels the mob off of you, at which point you should immediately switch back to your DPS presence.
Death Pact is going to be more useful to unholy death knights than to frost death knights, since an unholy death knight is more likely to have a ghoul out, but in a pinch, you may be lucky as frost to have ghoul off cooldown and ready to be summoned and sacrificed. Unholy death knights are also at a disadvantage when they use this if only because a ghoul is a good portion of their DPS, but when choosing between a dead ghoul and a living death knight, the choice should be pretty clear.
Death Strike itself isn't as potent as it used to be (or at least, it has a definite cap to its healing potential), but it can get the job done, especially for a frost death knight. Unholy death knights are going to have a bit of a harder time using it because it throws off their rotation -- but then again, when you're getting pounded on as a DPS, you should be breaking your rotation to focus on survival. If you can, consider fitting a Glyph of Dark Succor into your glyph loadout, at least at the 5-man level. That way, you can be your own healer for at least the incidental damage and occasional threat grab. 15% of max health healed per a strike is pretty great, especially if you have full bar of runes with 2 death runes ready to go. Of course, the big problem here is that you will generate a little bit of extra threat just by the damage put out by the strikes, so this method is probably best used to heal up after the tank has already peeled the mob off of you.
Army of the Dead is perhaps one of the most controversial and misunderstood death knight abilities, and with good reason. Since the ghouls it summons have the power to taunt, it can do more harm than good, knocking a mob out of position and causing it to spew a cone or other frontal attack on the DPS or healers and dragging it across the room, making it harder for the tank to pick back up. We've covered the ways to properly use Army of the Dead in a guide we posted a while back, so I'll just sum it up here. Due to the taunting ability of Army of the Dead, it should really only be used as a survival tool when a wipe is imminent, such as when the tanks and/or healers are dead and there are no battle rezzes available to get them back up. Then, sometimes, Army of Dead can save the day. I have seen it happen. Other times, it'll just delay the wipe for a few more seconds -- but hey, might as well try.
Lichborne We've covered the namesake of this column before, but it's a good tip to remember. Lichborne makes you undead. When you're undead, Death Coil heals you. Therefore, if you need some quick heals, you can press Lichborne and turn your spare runic power into health. It's a handy enough ability that frost DPS, at least, may want to consider using a free talent point to grab. It also breaks you out of a few crowd control methods, which can be handy.
Other survival strategies
Stop attacking It seems like a pretty straightforward point, but it's sometimes hard to remember. If you are in a position where a mob you are directly attacking is beating on you, stop attacking it. It won't make the mob leave off of you, but it will make it easier for the tank to grab it back. If you're generating more threat and your tank's taunts are on cooldown, you'll want to give them as much help as possible to grab that mob back by surpassing your threat.
Don't run around so much Every tank has probably had an experience like this. You've lost threat on a mob, but as you're trying to get it back, the DPSer who peeled aggro is running around like a chicken with his head cut off, trying to get away from the mob. Unless you are actively kiting the mob with snare spells (which, admittedly, death knights can do), it almost always a better idea to stay still until the tank can grab the mob back. If you run around, you confuse the tank, and the tank may have to drag any other mobs she's fighting around the room while she tries to grab the one you're taking for a jog.
In addition, running around heightens the chance that you'll run out of line of sight. If you've run around a corner or over the lip of a ramp, you have made it that much harder for your healer to target you for a heal or for the tank to target the mob. In almost all cases, it's better to stay still so that your tank can grab the mob and your healer can throw you a quick heal or two.
Know your position This is the companion piece of advice to the whole running around thing. Essentially, you should know what the battle is about. Does the monster you're fighting have a cone attack? If the monster is attacking you, did you just expose your fellow DPSers and the healer to more damage when they attack goes off? If so, your best bet may not be just staying still, but to run over and stand on the tank (not the healer!). That way, if the cone attack goes off, only you and the tank get hit. This also makes it easier for the tank to grab the mob back. Just be sure to run back to your proper spot behind the mob as soon as the tank has threat again.
The fine art of not taking damage
Of course, the ultimate way to survive in group DPS situations is often not to take damage. This may mean staying out of the proverbial fire, or it may mean not pulling aggro. This is sometimes hard to do if your tank isn't very good, but you can do some things on your end. Ask for a kill order if none is offered. If your tank knows which mob the DPS are targeting at a given time, it makes it much easier to keep threat. Watch your threat, using addons such as Omen if need be. Throttle your DPS. Throttling or stopping your DPS may seem anathema, but if it's a choice between being dead for 75% of a boss fight or throttling your DPS for 10 seconds while your tank gets established, one of those will put you much, much higher on Skada or World of Logs at the end of the fight.
Learn the ropes of endgame play with WoW Insider's DK 101 guide. Make yourself invaluable to your raid group with Mind Freeze and other interrupts, gear up with pre-heroic DPS gear or pre-heroic tank gear, and plot your path to tier 11/valor point DPS gear.