So your quest log is full of quests and you know there is good loot to be had, but the only group you can find needs a healer and your not specced for it. No sweat. With a little thinking ahead and understaning of your classes healing it can easily be done up to lvl 70. In fact, depending on your class, this will probably be how you instance up to lvl 70.
First you need to understand what your missing out on as a non-healer spec. Most healing trees provide: mana efficiency, spell power boosts, agro reduction, shorter cast times, sos spells, and occasionaly other knicknacks and spells.
As a non-healer spec you are probably not going to have a big mana pool. So a lot of what you need to do before hand is work out how to maximize your mana efficency. A really easy thing to do is keep a healing set stored in your bank. The easiest and cheepest way to do this is quest rewards. Every time you turn in a quest that gives you healing gear put it in the bank. Ive got a mixmash of cloth and leather healing gear currently in my bank. Don't buy anything unless its a really awesome deal. Your healing set doesnt need to be awesome stuff, just better than your dps spec stuff. I try to hold onto stuff with lots of stamina, intelect, spirit, mp5, and spell power. Other good things to keep stocked in your bank are pots (health, mana, and caster stat boosts), Enchanting oils (mp5 or spell power), good caster food, and lots of drinks.
Now on to the healing: It depends a bit on your class. So, I'm going to talk about the two healer classes that I'm familiar with and hope someone else can talk about Shaman and Paladins.
Druids:I have had no trouble main healing instances as a feral druid until lvl 40. At level 40 it got a little bit harder to do, but thats also when I started getting asked to tank the instance instead of heal. Win for me.
If your feral you REALLY need that healer set in the bank. Feral focuses on Stam, Agility, and Strength. So you are going to have a tiny mana pool. If your leveling as Balance then you probably don't need to worry about it as much. Your gear and tallent points can work well with healing.
Druids dont get the best returns from Spirit. So from my experience I prioritize my gear this way Stamina> spell power> intelect> mp5> then spirit. I recomend getting as much stamina and spell/heal power as you can. It lets you down rank your spells and take some hits. (druids have no good way to dump agro) Intelect and spirit are easy to find on gear so Ive found you can get a good bit of it easy. Mp5 can be a bit hard to come by but a little bit goes a long way if your downranking spells. When you get innervate then spirit becomes a bit more important becuase inervate is based on your spirit.
Druid healing: Resto tends to focus on their hots alot. As feral your going to use your hots and downrank healing touch as much as possible. As Feral you may be speced into Naturalist which reduces your cast time on healing touch. Using healing touch allows you to land a big long cast heal and have more time outside of the 5 second rule. {If you dont know the 5 sec rule -> it takes 5 seconds of not casting for your spirits mana regen to kick in and the 5 second rule doesn't start until your spell goes off.} So if you are outside of the 5 sec rule then the 3 sec cast time on healing touch is more mana regenerated for you. So at the start of a battle I will put a Rejuve and then a Regrowth on the tank. Then I wait to get out of the 5 sec rule and go: Healing Touch, Rejuve, then Regrowth. A resto speced Druid is probably going to go in the opposite order on the second half of that cast sequence. Watch the tanks health and down rank/anticipate your Healing Touches apropriately. Remember that
overhealing is wasted mana. It's better to not completly top the tank off than to over heal him.Thats pretty much it for Druid healing. Hope you have a good tank and don't pull too much because if you get into a heavy heal spamming fight your going to run out of mana fast.
Other general druid tips. If at all possible have another resing class with you. They are welcome to be as retnoob, enhancement, or shadow dps as they want. Just as long as they are ready to do the rezing. Battle rez is kinda worthless in any 5 man instance. Make sure the lock has SS on the class that can rez everyone and not you, the healer.
Priests: Similar to Druids, I haven't noticed a big differnce in healing until after lvl 40. From my personal experience Shadow Priests are one of the better off spec healing classes for PUGs. Main reason for this is PUGs are 'pretty ugly' and you normaly don't see a lot of focus fireing going on. Meaning that there are more mobs you can put VE and a SW:P on. I've run large portions of instances in Shadow form because all I needed to do was VE, SW:P, and the occasional shield. That doesn't mean that I was DPSing and didn't drop shadowform as soon as anyone needed a regular heal. Remember casting Shadowform = expensive mana use, but dropping Shadowform = free.
At level 40 your going to have Shadowform and VE. Your also probably running around with a lot of spirit (due to Spirit Tap) and +spell damage or +shadow damage. You may, but probably don't, have a lot of Int. So you don't have a lot of mana in your mana pool. I don't worry about keeping a big stock of healing gear in the bank like I do for Druids, but you should keep pieces around to replace your +shadow gear. +shadow will be of limited use as the healer of an instance, but don't sweat it too much if you have a little +shadow on.
Before lvl 40 there really isn't a big difference in Priest healing. All your missing is a little mana efficiency, shorter cast times, healing crit, +heal, etc... After 40 things change, but as shadow you can still be an awesome healer. I think of Shadow healing as being similar to Druid healing. Your going to use your HOTs alot with the occasional big heal.
Avoid using Flash Heal! You will run out of mana! You want to use all the +spell power and spirit that you are running around with to keep your mana pool full for when you have to drop some heals.
Priest healing: Before the fight: if you are going to fight a boss or mob that takes a while to kill, and have the time. Then go Shadowform and drink up. Let your tank gain agro and then apply SW:P and VE. If its a boss or a mob thats going to take a while then I put VT on. VE is great! Think of it as putting a downranked renew on everyone in your party for the cost of one SW:P. If your group is doing a lot of off tanking then give their mob SW:P and VE when VE's cooldown is up. If you have a bunch of mobs with SW:P/VE ticking away then the health will be rolling in.
BE CAREFUL if you have a mage polymorphing! They will get mad at you if you aren't paying attention and dot up the mob they are in charge of polying. Now pop Renew on people taking damage. Renew is pretty low in agro generation and with VE ticking most people in your party will be kept nicely topped off. If people need a bigger heal then you can shield them and let the hots tick or cast Greater Heal. You have to anticipate damage with Greater Heal, but it's pretty mana efficient and lets your mana tick (as long as your outside the 5 sec rule). Again: AVOID FLASH HEAL!!! I only use it when I'm desperate to get a heal off and even then I normaly PW:S and cast a downranked Greater Heal. (depends on the kind of damage the mobs are doing) If your tanks health is rocketing down then there is a good chance that your screwed.
Your not going to survive long if you have to spam heals.
Other Priest tips: Snipe runners. If you see a mob running away with low health. Wand it or even cast Mind Blast. You will get the killing blow and proc Spirit Tap which is more than worth MB's mana cost.
With the right group you can have some real fun as a shadow priest. Stay shadowform and get SW:P, VE, and VT ticking away. If you've found that your group can be left alone for a while. Then Mind Control something. If you do this, make sure your group knows what your are doing and is generally cool with it. By MCing a mob you are going to create a lot of agro so you want your group ready to kill your mob when MC runs out. Normaly this means everything else is dead. It's so much fun MCing that anoying healer or giving your group some killer buffs. As a bonus, if the mob that you MC has SW:P and VE on it, then it continues to heal your party. A final note about MC. If the mob you are controling is about to die. Then stop MC! (do this by right clicking the spell, jumping, or hitting Esc) If it dies while MCd then you can't loot it.