1) Sticky targeting: pick your targets and stick with them.
I'm not saying that cross-healing is bad. I'm saying that if you are healing a target and they clearly need healing you will generally want to stick with them even if they aren't the optimal target for your healing role. You've got two options: either you stick with your targets or you hope someone else will pick them up after you stop. If you leave such a target be ready to return with haste.
Most of the time you want to be able to distinguish between healing your own targets and cross-healing. If you are just healing randomly, then the odds are everyone else is also healing randomly. Healing clashes will ensue. You will probably need to communicate with your raid members to have some degree of territory assignments, even if you are mostly improvising.
2) Don't heal the first guy to take the damage nor the last guy.
A surprising number of people will subconsciously heal through people in the order they took damage (also applies to decursing). If too many people are taking damage to keep track of the "order" then you'll start to notice people healing the last guy to take damage (or just generally healing the people with lowest HP). Generally speaking you'll want to avoid healing targets based on *when* they took damage (a much better pattern is how to heal based on how urgently they need healing, but event this isn't always what you want to do).
3) Heal from bottom to top, not from top to bottom.
More people heal from top to bottom and left to right (alphabetically by group as they are listed in Grid) than pretty much any other healing order. To avoid healing same targets, try healing from from bottom to top or right to left. Or alternatively try to avoid healing in any particular order, instead picking each target separately.
4) Heal based on class.
You will want to heal based on class. Some classes got a lot better emergency abilities than others. Ideally you'd heal based on who has least savers available to them (and least HP/resistances) but in practice absorbing all related information is next to impossible. I'll claim that it's generally better to just heal based on simple class/group rules. A few suggestions for high-priority healing:
- Enhancement shamans
- DPS warriors
- Mages (Iceblock can be very strong for some encounters, though)
- Healers (low'ish HP, no judgement of light healing)
5) Heal based on group.
- Try if it's worth prioritizing group 4 (and 3) when there are no strict healing assignments.
- Prioritize groups which aren't fully ranged nor melee with spells suitable for such things
- Prioritize groups which *are* known to be in same location (typically melee groups) with heals suitable for such things.
6) Make conscious decisions regarding tank healing (especially with heal-over-time spells).
If your HOTs can help tank healers drop the occasional direct heal on raid, then throwing hots on tanks is probably a good thing. If your hots can occasionally help save tanks from death them dropping HOTs on them is probably also good. If your HOTs don't accomplish either of these things then don't drop them on tanks.
7) Heal yourself first.
You never know if someone else has cooldowns which can save them event at low health but if you neglect yourself at low health you are basically throwing the ball to other healers. Priests will force themselves to use binding heal.
Additionally: it is very often a good choice to use items (potions, stones) or cooldowns to save yourself while keeping up the heals on others.
Heal based on who needs to live
Some deaths can easier be fixed with a combat res than others. It's often a good idea to heal based on who needs to live rather than who stands greatest risk of dying. I could state this another way: heal those who need to be healed rather than those who need heals.
9) Be aware of PW:S
This is probably the single most important non-encounter specific thing to avoid overhealing: don't throw hots on targets with PW:S up (this is true almost regardless of their current HP status). On the flip side do throw hots on people with Weakened Soul. If your raid frames aren't showing you these effects then fix your raid frames.
10) Don't mask problems
I can't believe I originally forgot to say this: try healing the guy who tends to die a lot, but never do this in such a fashion that will mask the problem. Minimally you should mention to other healers that you'll be keeping an eye on the unlucky fellow. Preferably when there's a good reason for someone to be in high danger then they will receive special attention as a part of the strategy or strategy will be altered such that they are no longer in danger. If there's no good reason for them to be in special danger then perhaps there's a bad reason.
Put another way: it's probably better to solve the problems at root rather than try and be the silent hero.