I've healed, and I've tanked. I tank less often than I heal these days, but that's just a factor of what role my guild needed me in when entering the expansion. I love my tanks. I know a good tank when I see one in a pug. I also try my best to work with them to make their jobs easier, because there's more to healing than casting spells at health bars.
1. Stay behind the tank.
If you are unfamiliar with the instance and might aggro something, stay behind the tank rather than running ahead. I think Ram wanted to kill me after I bounced ahead up the road in my first PoS run and was suddenly in combat with mobs that I thought would've had a longer aggro range; don't run ahead of the tank unless you're absolutely sure of what you're doing. If you do aggro a new pack of mobs, that's your own fault. As a note for pugs, be wary of DPS that look like your party's tank as they might run ahead, aggro a pack, and then dump aggro on you as you start healing.
2. If you have aggro on a melee mob, run to the tank.
Don't run away from the tank. They are, for the most part, melee characters with a limited range on their taunts, and they're working at the same time to maintain aggro on the dps targets. Do your best to avoid cleaves or breath attacks from other mobs when doing so.
3. If you have aggro on a caster mob, try to LoS it to the tank.
LoS (Line of Sight) is your friend; use it to hide from caster mobs that are trying to shove fire balls up your robe. Preferably have the LoS bring the caster past or near the tank, so that they can more easily pick it up as it runs.
4. If the tank is using LoS to pull, you need to LoS, too.
Stand with or slightly behind the tank. Any heals (hots) that occur while the mobs are running over will establish aggro on yourself before the tank has had a chance to hit most or all of the mobs, so you need to be proactive in helping to bring those mobs to the tank via LoS.
5. Don't stand in cleaves or fire novas or breaths or similar tank-only effects.
That's not aggro. That's standing in bad stuff. Don't do it. Reposition yourself so that you are not in the cleave/breath/whatever. If your tank is going crazy and dragging stuff all over the place having it cleave or breath on the dps, then smack them upside the head. If you are moving yourself and jumping into the cleaves, that's your own fault.
6. Remind them to hold up if you really need to drink before a pull.
Something as simple as "OoM" or "Mana" in party chat should be enough to halt a tank in their tracks. If there is lots of party-chat discussion, you may need to add emphasis to make sure it's not lost in the scroll of chatter.
7. You will have aggro on anything the tank hasn't been able to touch.
Healing aggro is not a problem on mobs that a tank is able to work on (melee range), but during an initial pull of mobs, you will very likely get aggro on something. The "initial aggro" generated by the player who initiates combat with mobs is not very much, and healing will overtake that threat quickly. This includes situations as, listed above, need to be LoS-pulled. This also includes mobs that some dps happened to get too close to, and the tank may not be aware that they've been aggroed. Run any aggro to your tank, or position yourself so that they will cross the tank as they are running to you; if your tank is at all attentive, they will rescue you.
8. (Druids:) Innervate your Prot Pally.
If they're OoM (or near to it) and your mana is fine, innervating a prot pally tank is a great way to reduce downtime during chain-pulls or, if extra stuff is pulled before they can drink, to make sure they can hold aggro against everything. An OoM pally-tank can't keep aggro against high dps, and your innervate will overflow their mana pool!
9. If you had a great tank, thank them.
Sometimes I get thanked as a healer for what I thought was a light job. I smile and say, "My job was easy... x was a good tank" (or something to that effect). Sometimes I have an insane job, and I get thanked, and I know that the tank should be thanked, too: such as when I ran a heroic HoL with a prot pally who had 25k health, and we accidentally pulled the first boss (while charged) and an entire pack of trash at the same time. The tank was very low in gear level, but he picked it all up before it could kill me and used his cooldowns to stay alive while I poured heals on him and the group. I knew it wasn't all me: that tank was a great tank in spite of his gear, and our dps was wise enough to let him build aggro. It was the best pug group I've ever been in, and we had no deaths through the whole run. I made sure that tank knew I appreciated it, too. Tanks don't get thanked enough.
10. Communicate with your Tank.
Tielyn commented with this great addition.
"Communicate with your tank. Warn them if your gear is a little light. Or tell them they can go all out. There is nothing worse than landing in a random PuG with a tank that says, "I'm a gonna chainpull, kk?" That is your one chance to speak up and tell 'em 'No, I don't think I can handle it.' If you don't, it really IS your fault.
If they insist on pulling when there's no LOS to you, however, it's probably their fault.
Conversely, when I tank, I look at my healer. And I ask him or her, 'I see you've got 22K mana and what looks like mostly T9 gear. We good to speedrun this puppy?'"
4 hours ago