From a WHM perspective.

1) Either keep pulling or stop. Don't stop to OP or Flash 2-3 times and then keep going. You're taking more damage, I think you're stopping and going to Regen you and start Holying. Flash while running through a group of mobs and then use Shield Lob/Tomahawk to get aggro on ones not hit by flash. This will also keep DPS from ripping mobs off you as they'll start attacking when you stop as well.

2) Use your cooldowns. I have gotten kicked from a run for telling a PLD in AK HM that he needs to actually pop buffs when he does the whole hallway + room for the 2nd trash pull. If I have to blow Divine Seal, Presence of Mind, Swiftcast + Cure II, and Bene and the tank still dies after rotating through those, the tank isn't popping enough buffs. I can't heal 3k+ of unmitigated incoming damage when I only have 2.6k Cure II's.

3) Wait for Stoneskin. I will try to Stoneskin you when the last mob is at 25% or I'll Swiftcast + Stoneskin II after everything dies. But sometimes you run around a corner and I have to start casting all over or Stoneskin doesn't stick on everyone and I have to cast it manually. I don't want to pull aggro off the things you're about to pull, you don't want chase them. So if you see that party arrow on your screen from my casting, just stand still for a second or two and it'll be okay.

4) Don't pull what you can't handle. I know people like speed runs, but if you can't keep hate on a big group or you're just barely on ilevel requirement for the run. Then stick to smaller groups that you're comfortable with handling.

5) Watch a video before the fight. Not as big of a deal for a dungeon, but a tank is supposed to lead the group. So even if it's your first time, watching a video will help you understand what to expect so you don't blindly lead your party into danger or get surprised by a boss mechanic.

6) If you're in a 24 man raid, for the love of everyone's sanity, don't fight the other tanks for aggro (I mean, sometimes you accidentally rip the boss off another tank, but I mean doing it intentionally). I was in a WoD last night and I saw a tank on Cerb's butt provoke after the tail AoE came out, and Cerb cleaved right away. That tank literally 1-shot all 10 melees in that raid and a couple of healers.

7) Compensate for your party. If you're running 2 melee, try to have all of the mobs facing the same way. If you've got 2 BLMs (and you feel comfortable doing so), pull big so they can utilize their OP AoE rotations. If you're doing a low level roulette and 2/4 people are new, ask the 4th person if they wouldn't mind doing a full clear and help the lowbies.

8) It's okay to make mistakes. Everyone does. Sometimes we wipe and/or people die and that's okay. Just learn from the experience and try not to take it out on everyone and I promise it'll be okay.