General

To simplify, the goal for a healer is that nobody dies. A healers job should be to always identify the dangers of any boss/trash mob in an instance, and do his/her best to keep people alive.

A healer is however not of much use, if he cannot keep an eye on both the raid and the Main tank. A healer also has to keep track of boss ability timers and his own health & mana.

These things are easy, and it seperates skilled healers from less skilled healers.

The spells
I'll now go through all the priest healing spells:

Greater Heal
The greater heal is the most mana efficient single target heal a priest has, and you should use several downranks of it. I personaly use rank 7, rank 3 and 1. The lower ranks are used on randoms, that are not in a hurry to be healed, while the higher ranks are pre-casted at a tank before he gets hit by a boss' special ability.

Renew
Every priest in a raid needs to have his renew up on the maintank(s) at all time - even the ones healing randoms. Renew can also serve as raid-healer, if you know someone is gonna take damage in the near future.

Prayer of Healing
As priest you must allways keep up your own group. Prayer of Healing is very mana efficient, and should be used at any possible occasion (when needed). Prayer of Healing can be downranked to heal less wounded groups.

Circle of Healing
Not all priests have this amazing spell, but it outputs a massive heal, if used correctly. When cast at a person, it will heal everyone in his group in range for ~1000 with proper gear. That means, that you can easily heal for 5000/global cooldown for almost no mana. Used together with PoH, this will be a large heal to keep the raid up. (This happens more often than you might think). CoH is mostly good in 25-men instances for healing groups during AoE events.

Binding Heal
Most priests use this spell far too little. It took me some time too, before I learned how good it is. The main rule is, that you should use this spell whenever you have little HP/take damage. Just find somebody that doesn't have full HP and use binding heal till you have full HP. Binding heal has a high heal/sec, and an acceptable mana efficiency. Using flash heal on yourself should never, ever happen.

Flash Heal
Flash heal has a bad reputation for it's low heal/sec and bad mana efficiency. It does however have its good sides; If your tank needs a quick heal, you have your flash heal. It is way faster than your Greater Heal - use that.

Prayer of Mending
Your goal with Prayer of Mending is to make it bounce 5 times before the cooldown is ready. Prayer of Mending is not really a target heal, but you must use it on your main tank every time you can. You can also use it on people that are gonna take damage. Examples of events where this is most useful are Bloodboil, Gruul, Lurker and RoS. If PoH hits yourself, don't feel bad to bounce it along by using Shadow Word: Death.

Power Word: Shield
PW:S is rarely used in raid, since it has a very bad "heal"/sec, and it prevents the warriors and druids from gaining rage when hit, and prevents paladin tanks from dealing damage, thus threat, with Holy shield, Retribution Aura, etc. If you must use it, use it on mages or warlocks before an AoE pull - not in the middle of it.

Fade
This is the spell that every other healer envy us, so use it! A priest should never pull aggro. The highest rank reduces threat by 1500.

Dispel Magic/Abolish Disease
Use these spells whenever needed. Dispelling a magic or a disease can be very critical - don't count on the paladins to dispell fears, dots etc.

Builds

There are two specs for you to choose from - either you go for improved Divine Spirit or Circle of Healing. A specc with Divine Spirit without improved Divine Spirit is not acceptable nor good.

Imp DS-build: http://www.wowhead.com/?talent=bxg0zhxzbZfxxcc0qVhM

CoH-build: http://www.wowhead.com/?talent=bxT0zhZfxxcfMqVuVo

The most important things are inner focus, meditation, healing focus, divine fury, improved healing and empowered healing.

Mana

Controling your own mana and the ability to regenerate mana while fighting is very important if you want to do good as a holy priest in raids. You must at all times be aware of the five-second rule (FSR). To briefly explain the FSR, the rule means, that you do not get mana from spirit-regen 5 seconds after casting a spell that casts mana. The more spirit, and the less mp5 you have, the more benefit you will get from being aware of the FSR.

To put an exampel: Mr. m00 has a 130 mp5 within the FSR, while he has 500 mp5 outside it. m00 doesn't have any mana potions left (shame on him), so when he is at 50% mana, it is about time to think about regenerating. He heals up everything that needs a heal, and throws a renew on the tanks. Then he doesn't do anything for five whole seconds. At that time, m00 activates the trinket that gives a 300 spirit bonus (http://www.wowhead.com/?item=30665) and inner focus. Now m00 has an mp5 of 700 for 20 seconds. With inner focus up, you can chaincast the highest rank of greater heal on a tank, but cancels it before it is cast if it is not needed. When m00 has used his inner focus heal, he is still outside the FSR, because his heal did not cost mana.
When you feel like you have enough mana, just start healing again. The point is, that m00 has regenerated 3-400 mana while renew is ticking on all the tanks. A priest should always depend on spirit regeneration when he can. The ability to fully control the FSR is one of the things needed, to be a good holy priest.

Enchants and consumeables
The best consumable food for a raiding holy priest is without a doubt the golden fish sticks (44 healing, 20 spirit).
The best battle elixer is the eilixir of healing power (50 healing).
The best guarding elixir is the elixir of draenix wisdom (30 intellect and 30 spirit).


When it comes to enchants:
Head: Thrallmar/Honor hold reverved headenchant (35 healing and 7 mp5).
Shoulder: Aldor/Scryer healing enchant.
Back: If you have aggro problems (shame on you), get sublety (-2% threat)
Chest: 15 spirit or 100 mana.
Bracers: 30 healing.
Hands: 35 healing.
Pants: Golden Spellthread (66 healing and 20 stamina).
Rings: 20 healing.
Weapon: 81 healing.

Congratulations, I have seen many nice guides from you, so when I next log on or briko. I feel you need to be promoted xD