DKs in general:
Outlast them, especially blood. They will heal if they can hit you, which simply means don't get hit. Start with a stun, then gouge, then dismantle them. Then stun them, maybe another gouge, and evasion. At this point you have nothing, and I'm sure they have immuned a stun, so DRs are up. (If they trinket or immune one of these just move on, you get the idea) Once everything is used though, sprint away, and try to get a smokebomb on your location before they grip you, to avoid said grip. Reenter stealth some way or another, let DRs finish, and repeat! Fact is DKs can't put out much damage while healing themselves, and you can heal through that. If you can outlast a DK while not getting hit, you will win. This might take a while, but it is much less futile than hitting a healer. Still, if you are in a BG or arena, don't bother with a blood DK, tanks don't do damage anyway.

Ferals:
These are considerably tougher. You want to open on them, since you need every advantage possible. They have no way to remove stuns aside from trinket, and they need to go out of combat to heal. These are really their only disadvantages, so use em. Anyway, try to keep the feral in combat. You don't want them restealthing, and since they have faerie fire on you, you can't restealth without CoS. Try to save CoS for when they root you and go to heal. Then vanish and get to them (SS if sub). FERALS HAVE AN ENRAGE. Memorize what it looks like, i believe it creates a red glow on the paws, and the buff is the head of a cat-like animal, but I could be wrong. When you are low on energy, just kite them. keep them in combat via throw, but there is no sense in fighting if you can't use any abilities. Also, when they cyclone and heal, that is when you trinket.

For both, control is your greatest advantage, so use it. Don't get hit unless you are hitting them harder, keep up recup (esp if you are sub), and force trinkets or immunities before bursting them.

Not sure what spec you play, and that will make a difference. I play assassination, so here is what I try to do against high armor targets:

1. Open with sap if possible, to get the effect of blackjack. Garrote is your best opener, because it blackjack is essentially wasted if you use cheap shot. Garrote also puts a long term bleed on a fellow rogue, so he can't restealth for a while.

2. After garrote, I get one mutilate, which almost always applies crippling via deadly brew. Since I use cold blood, it also crits, so I now have 4 cp's because of seal fate.

3. I use a 4 point rupture. This applies a second bleed to the target, which is important because those bleeds ignore armor and will continue to deal damage if he trinkets and tries to get away from you. Plus, since blackjack is still up, he is not doing a lot of damage.

4. 2 more mutilates, or maybe 1 mutilate and one shiv, depending on how my deadly poison is stacking.

5. 4-5 point KS. As the KS breaks, I'll use dismantle, then an envenom for the cp's I built during the KS, if it is a DK. If it is a feral, I'll skip the dismantle, and use recuperate instead of envenom. He usually hasn't done much damage, so the recuperate is more for the damage reductuction (you are specced into imp recup, right?), than for the heal.

6. At this point, you'll notice that I've done nearly 15 seconds of damage without taking any in return. With 2 bleeds and a stack of deadly going, the target is usually down to 50-60% health, and dropping quick.

7. At this point, I try to make sure of 2 things: first is that I always have rupture up, second is that I always have recup up, again for the damage reduction. If those things are going, time to use envenom. I'll use evasion or CR just to keep him frustrated, and possibly lure him into a desperation mistake.

Regardless of opponent, bleeds are key pvp. Make sure that the mage or hunter that is trying to kite you is taking damage while he's running away. Make sure that the rogue that wants to vanish and reopen on you can't do it. Make sure that the healer trying to cleanse your poison still has to use mana on a HoT to counter your DoTs. Use vanish as an offensive cd, to give you another stun if you need it.

Again, these are some tips for assassination rogues, and may not work for other specs. Don't forget about vendetta, it helps a lot. I have macro'd cold blood and blood fury (orc racial) to mutilate and envenom, so they pop whenever they ate off cd.

Finally, it is important to know when not to react at all. Just last night, in a 2v2 match, it was down to me and the arcane mage. Arcane doesn't kite or survive like frost, but arcane blast hurts really bad. We were both at about 50% health, and he sheeped me. I still had my trinket availble, and thought about using it so that he wouldn't run away. But I noticed I was near a pillar, and have you seen your health regeneration while sheeped? It's HUGE! So I healed up while in sheep form, when it broke I ducked behind the pillar to break LoS, and restealthed. I then opened up on him with both vanish and my trinket available, about 80% health. Tore him apart in about 10 seconds.

The point is: sometimes controlling the fight means doing nothing and letting your opponent beat himself with his own mistakes.

Hope that helps, and good luck