Food: The art of keeping well fed
Assuming you've kept your cooking skill up, a good piece of stat food can be the easiest and quickest way to give yourself an extra edge. If you don't have your cooking skill up, chances are your have a friend or guildmate who does and wouldn't mind cooking your food if you provide the raw materials.
For DPSers, there is generally only one food you need to worry about: strength food. In the Cataclysm era, that means you want Beer-Basted Crocolisk. There are two major sources of the Crocolisk Tails you need to make this. You can kill the Crocolisks in Tol Barad (handy since they're also the subject of a daily quest), or you can head out to Uldum and kill Crocolisks along the banks of the rivers there. I can usually grab a little over a stack of tails clearing up and down both sides of the forks in the north part of the zone. It is worth noting that if you are having trouble hitting your hit or expertise caps, you can try carrying a stack of Grilled Dragon or Crocolisk Au Gratin. That said, you should just use reforging to hit those stats, as the extra strength from strength food will do you a whole lot better.
For tanking, you'll probably want to focus on mastery food, although you always have the option of going after parry or dodge food. (Go with whatever stat you have less rating in, if you go that route.) In Cataclysm, your best mastery food will be Lavascale Minestrone. This will require you to fish up Lavascale Catfish. Unfortunately, this fish do not travel in schools, so you'll have to try your luck with fishing in normal waters. Deepholm and the rivers of Uldum seem to be the best place to fish these up.
Mushroom Sauce Mudfish and Blackbelly Sushi are your dodge and parry foods, respectively, and both can be cooked from Blackbelly Mudfish. You'll find schools of Blackbelly Mudfish up and down the rivers of Uldum. As you might suspect, this all sort of points to Uldum as an awfully nice place to farm food for a death knight. Spend a night between DPS dungeon queues killing Crocolisks and fishing in the rivers while you wait for them to respawn, and you'll be stocked on delicious food for weeks to come.
If you want a wild card option for your food, you do have two other choices. Fortune Cookies are made using Mysterious Fortune Cards and automatically give you 90 in a stat based on your current spec. Of course, Mysterious Fortune Cards can get pretty costly, especially if you have to buy them off the AH, so keep that in mind.
Your other option is Seafood Magnifique. It'll provide a relevant buff to all members of your party or raid. It does, however, require some extensive fishing. You'll have to grab not only Lavascale Catfish, but Highland Guppies from Twilight Highlands, and Fathom Eels, which you can find in the oceans off Uldum or in Tol Barad Peninsula. It comes from a guild fishing achievement, so you'll need to be in a guild that loves fishing (or can tolerate it long enough to get the achievement) to get it. It's also bind on pickup, so you will need to be the cook in question to make them.
As far as when to use food, it's relatively easy to farm up, so I don't feel too shy about using it. Generally, if I'm doing a heroic dungeon, I will chomp down. If you want to be a little more frugal, you can save eating food until the raid level, but if you're doing anything beyond heroic dungeons, you'll definitely want to keep a couple of stacks of food ready to eat.
Flasks: They aren't just for whiskey
Flasks are a handy thing to have because not only do they provide a buff that's even larger than food, they last through death. As a trade-off, they are unfortunately harder and more expensive to create, being a product of lots of herbs via the alchemy profession. That said, on my realm, most flasks are cheaper than 100g each on the Auction House, and there's also the option of gathering the raw materials yourself and finding an alchemist with elixir mastery who's willing to let you keep any extra flasks they proc. In addition, you can sometimes get the flasks you need by tanking heroic dungeons during a tank Call to Arms, though that's much more random in success rate.
On the plus side, flask selection is pretty easy. DPS will take Flask of Titanic Strength, while tanks will want a Flask of Steelskin. If you're lucky, you may even have an alchemist along on raid who can drop a Cauldron of Battle, which will provide you with a Flask of Battle. Since the flasks last only one hour, you may need at least two per raid. Flasks are generally expensive enough that I don't bother with them for heroic dungeons, but for raids, they can offer just the edge you need.
Keep it together with bandages
The use of bandages often seems to be a lost art. In vanilla raiding, I remember well the art of ducking behind cover and quickly bandaging up because the healers had to focus their healing on the tanks. This, of course, became much less of an issue in Wrath raiding, especially, because AoE damage and high mana regen rates meant healers both could and needed to heal everyone, DPSers included. Cataclysm, with its promise of tougher boss fights and lower mana regen meaning more meaningful choices for healers, teased of a return to that model, although in execution, some believe it's fallen more than a bit short.
That said, a little emergency healing, especially with the nerfs afforded Death Strike and the Lichborne heal trick, is never a bad idea. Dense Embersilk Bandages can be made with Embersilk Cloth, which drops rather abundantly from humanoid enemies in Cataclysm zones. Unfortunately, you can't use them if you're hit with a bunch of DoT spells or in a fight with lots of AoE damage, but if you're hurt and the healer can't help you, ducking out of sight to bandage up can be just the thing to keep you in the fight. I especially enjoy them in PvP. If I've just killed an enemy or evaded them by turning a corner or jumping down somewhere else, but I'm deep in enemy territory, sometimes it's a lot easier to quickly bandage up and move on, rather than sit and eat and risk being ambushed.
The use of potions is, if not esoteric, one that doesn't seem to see much wide use these days. It's somewhat understandable. Potions are sort of the ultimate consumable. You use them once, the effect takes place immediately, and then it's gone, and whatever gold or time you spent getting the potion is gone along with it. In addition, even the old standbys of healing and mana potions have gone out of favor, since even the most recent versions barely register in the modern era of huge health and mana pools. While you can sometimes mitigate the costs by getting a sympathetic potion master to make you some potions and give you any extras he creates, even that's a shot in the dark.
That said, potions still have their place. Golemblood Potions provide a huge burst of strength for DPSers, while Earthen Potions provide a similar burst of mitigation for tanks. Since you can only use potions once per battle, it's generally a good idea to use them when the need is greatest, often to get through a particularly hairy phase of a given boss fight. The practice of pre-potting is also popular with progression raiders -- that is, taking a potion just before the fight, then taking another one later in the fight when the cooldown timer has reset.
Again, with the expense of potions, using them regularly has a strong chance of putting you in the poorhouse. Therefore, I certainly wouldn't blame you if you use them for progression raiding and progression raiding only. Of course, it's always nice to have a few on hand just in case. And if you're in a regular raid group, you'll probably have direction from your raid leaders as to when and how they should be used.
One overlooked use of potions is the low-level utility potions you can get. As someone who got the Legionnaire title back in vanilla, Living Action Potions have long been both my savior (when I use them) and the bane of my existence (when an enemy uses them), but having one ready to use can be the difference between a flag capture and a flag return in many a Warsong match. Swiftness and Invisiblity potions can also have uses for sneaking around battlefields.
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.