One can certainly find a lot of basic information on DK raiding on many sites. DK info and the erstwhile Skeleton Jack (RIP) are examples.
You don't need to read this post. Go somewhere else. No, really.
Still here? Ok, well, if you insist, I'll post some infomation on DK basics for any readers not uber-informed on raiding with a Death Knight.
Consumables are a bit of a pain. They take up bag space. They cost lots of gold. They cost lots of gold. They don't make a huge difference, and they cost lots of gold.
Actually, let's get this straight. A FULL night of progression raiding (say, 4 hours or ~15-20 attempts) with maxed-out consumables can cost you in the ballpark of 400g. Most nights will be much less. Difference? At least a 5-10% overall buff. On progression, that difference will make or break you. And since this post is not about gold-making, I'll just say, Naga, please. You can make enough gold in a few hours of farming, or less if playing the AH for a whole week of raiding. It's worth it.
On with the show.
Good/Better: Elixir of Mighty Strength/Elixir of Protection. You get 40 strength, which benefits you and your pet(s), which also receives increased synergy with Blessing of Kings. If you have Bladed Armor in your spec, the protection elixir gives you another 20 ap or so, the equivalent of a decent enchantment. This combination is usually recommended for farm content, because it's not the best buff available and is lost when you die, but it does give quite a significant boost to your stats and therefore, dps.
Best: Flask of Endless Rage. This flask is generally acknowledged to be the biggest boost you can get. It's expensive, but fortunately lasts an hour and persists through wipe after wipe after wipe.... the 180ap is nothing to sneeze at.
Honestly, don't bother with the armor penetration/expertise/etc specialty flasks. They just don't give the return for benefit vs cost. Some folks argue that the Elixir of Armor Piercing is worth it (even a few recent posts on Elitist Jerks), but on any and all decent tests/theorycrafting to date, strength still comes out well ahead of arpen, especially since the nerf.
"Ghetto Food:" if you have to eat something and you have meat lying around: Dalaran Clam Chowder, Mammoth Meal, Shoveltusk Steak, Worm Delight, Worg Tartare, etc can provide a small buff. I tend to use these in heroics if I have the meat on me. You certainly won't get much out of these, but they're better than nothing. Don't waste your northrend spices on them.
Better: If your raid provides it: Fish Feast. Many if not most guilds provide this food for the raid. It's a nice buff, and if it's free, hey, don't complain! 80 attack power is pretty solid. I will often resort to this food for farm content.
Best: Dragonfin Filet is the best food you can get, since the strength buff is generally considered to be worth about 100-120 attack power. But it does get a little spendy (or time intensive, if you fish).
Don't forget your pet: If you're unholy, buff your pet with Kibbler's Bits, the (buzzard) meat is usually silly cheap on the AH.
Before pull: If you've specced into Bladed Armor (currently, both blood and unholy raid specs use it), you should be using an Indestructible Potion seconds before the pull. That gives you ~100 attack power for 2 minutes. This buff is only calculated after your strength value changes from a strength proc (Fallen Crusader, Sigil, etc) or weapon swap. Using a potion shortly before the pull allows you to use one more potion after the pull....
After pull: use Potion of Speed when you need burst, preferably during bloodlust/heroism, since haste effects stack multiplicatively. Other potions, including the strength one - this time - are inferior. 500 haste is just very very powerful.