Healing is the hardest part of WOW, but in my running instances at 80 I've run into some problems in all the roles.
If you are lucky to get into a group as a DPS, you prolly don't really need to worry about much unless your healer and tank suck. Having done both and sucked as both, I know what they look like.
If your tank can't hold aggro and keep the mobs off you, then your tank sucks. Some fights its unavoidable that a mob will aggro you, but does your dps respond fast enough to get them off you? Does your tank come back and scoop them up? Those are the questions you should be asking. For example, I was running Heroic Gundrak. I inspected the tank and it looked like he should've been able to do it, but at the Serpent boss, I kept dying...The boss summons snakes that the tank must pick up, which he wasn't. So I switched to pally, we found a healer, and we finished the instance. Some things I learned, however, know the fights. Use wowwiki.com. I went into the instance not knowing nearly enough.
If your dps can't beat the mob fast enough, your healer, no matter how good is going to run out of mana (OOM). Use the damage meters. I'm sorry, even a fresh lvl 80 should be doing a 1000 dps. For example, in heroic Violet Hold, I ran with a group that was barely breaking a 1000 dps. Though I was going to dps, I noticed that the pally had only 17,000 health (as opposed to my 22000)and asked if she was going to put on her tanking gear. Then as we got to the 2nd boss, the same pally was only doing 500 dps. We died multiple times, and all because the healer ran OOM. But the healer ran OOM because the dps wasn't doing their job.
Know the fights, watch the healer aggro or even aggro that squishies get and be prepared to cast a Distracting Shot (Hunter spell), Roots (Druid Spell) Hex (Shammy spell), if the squishie somehow gets aggro. Tone down on the Area of Effect (AOE) spells until the tank solidly has aggro. I generally don't use AOE until after the first mob dies. It totally insures that the tank has aggro. Use a CC even if the tank doesn't call for one. If a fight goes horribly wrong, Hex somebody (just don't Hex the Tank's target). If you know the fight includes Poison lay down a Cleansing Totem (Shammy Spell),even if the healer can take care of it. If you know that some part has some major AOE and you can heal, throw in a group heal or heal yourself occasionally. You're one less person the healer has to worry about. Frankly, I've been in too many instances where the healer is so focused on the tank that they'll let me die or not notice me dying because they are so worried about the tank. Don't assume they're watching your health bar. Some fights (think Loken's Lightning Nova) the healer's job is to get ppl up to full health before the next one. Helping the healer can be just as beneficial as helping the tank. Use an assist Macro so you are always targetting the tank's target. Here's mine:
/assist [insert tank's name here].
/castsequence Hunter's Mark, Serpent Sting
/assist [insert tank's name here]
/castsequence Faerie Fire, Moonfire, Insect Swarm
The assist macro works wonders....With more than one mob, just tap it when the first mob goes down and you're switching to what the tank is attacking. No delay, no inadvertant aggro.
Get out of the fire, the purple rings, the blizzard. Attack from behind and limit your burst damage. Use Rupture over Eviscerate or even Envenom (all Rogue abilities) early in the fight. You can still use the other 2 but wait to insure the tank has aggro. Use Feint (rogue abilities) to lessen your aggro. Be prepared to Vanish.
Those are my basic pointers from running Heroics and raids this week.
Next post? Using your procs....Use them...