its hard to be the first to follow a poet as grand as the mighty wind, but here is my best.  

First I view the roll of the preist to be one of primary support.  Its like a good jock strap, with out it your balls start to hurt.  similarly with out a preist, you have a great chance of getting your nuts chrushed.  Sure you can get by with others, (shamans) like breifs, but for hardcore play, whether basketball or dungeon running, you should use the right tools for the job.

Healing:  I heal differently depending on the situation. 

Small Raids as only preist:  (5 and 10 man) my main goal here is to keep everyone alive.  (obvious I know)  but in order are; tank, mage/ warloc, shaman/druids, hunter/rouges.  If the tank dies there is a much higher chance that we all will die.  I try to keep the dps up next, because its dps, and they can't heal as easily, or evade the conflict.  shamans/druids can always take care of themselves when they get too low.  rouges can vanish, and hunters can play dead.  

Small raids with another preist:  same as above, but I like to either divide duties up as either a group 1/ group 2, or MT and dps healer/ disspell and other healer plus dps.  this is a little more complex as it depends on the instance and ability to withstand damage of the tanks.  plus the tanks crowd controll skills play a great roll here.   I have been with some tanks where they are the only target you have to heal, while some let the cloth wears wet their tunics.  duties can ushually bedecided among the preists, and/or other healers.  

Things I look for:

Shielding prior to pull:  often it is best to shield the warrior prior to the pull for the extra health he may need for the initial fire he is going to take.  However this also means that he is not going to generate the same rage as fast, and therfore may be slower to gain agro.  I rarely find this to be the case, but some warriors swear by it.  things to remeber here is to let the preist shield prior to the pull, that way the preist doesn't take on agro.  this can be the same for a mage or warloc who may need to be shielded if the warrior has a problem holding agro. (too low level, or just a shitty warrior, or the dps never give him a chance to get it to begin with)  

Reducing preist Agro:  The reason a preist is often attacked, is because they generate agro with every heal they cast.  If a mob is bashing on a tank, and a preist has to spam heal, he is cranking up his agro as fast as he can to save the life of the tank.  The reason the preist isn't attacked is beacuse the warrior has tools such as taunt, and revenge to keep the mobs attention.  Now if a mage is the one needing spam healed, the preist is not just saving the mages life by healing, but also by pulling agro.  As most cloth wearers don't make great tanks, this can result in a dead preist.  I know the natural instinct for anybody when attacked is to fight back, but sometimes it is best to stop blasting away, and let the tank, pet, void, or a shaman gain the agro.  

Wasting Mana:  I know everyone likes to rock with full health, but sometimes it is ok to have 50% in order to make sure the tank is still walking at the end of the battle.  Often you will notice I am one of the last people healing myself, and often rely on the shaman for my health.  I figure that if everyone else is doing their job, i should be ok.   I try to limit my use of offensive spells except when I have plenty of mana, over 50%, or when it is evedent the team is going to win and we will break after the battle.  (need to rez, or buff) I often think that the use of dps from a preist could make some runs go faster, yet I know others in the group don't often share this belief and feal more comfortable with the presit having mana just in case.  I try to conserve mana by waiting and using greater heal v. flash heal, but am often not allowed to because halfway through the cast, someone else has healed the tank and my greater heal only does 1000 health or &lt; a flash heal equivelent anyway.  Or I have to cancell my spell altogether.  This doesn't only waste mana of the preist, but also the shaman or druid who just healed.  Plus it waste time.  this time and mana could have been used healing the mage or rouge that was forced to disengage to save thier life.  or the preist could have used that mana ot mindblast or holyfire instead.  I have added a message when I cast my greater heal to alert people, yet many people don't realize it.  

Shackle:  Out of consistency I try to shackle the most right target in a mob of multi targets.  There is no advantage for this, other than to build an understanding of what target not to shoot.  when a target breaks shackle, I will always reshackle as long as there is not a tank to pick it up, or it was broken on purpose.  I know when poeple see a ghost break shackle do to a scatter shot, or cleave that went too wide the natureal instinct is to pick it up and protect the preist.    This is one case where I like to not be protected to that I can re-shackle it and you can continue to engage the targets that are already active.  By not attacking the ghost/undead you save the preist mana from wasted shackle that gets broken right away from your attack, and you maintain focus on the the other targets to drop them faster.  

Mana regeneration:  I know this can suck waiting for the preist to always drink.  I have tried to change my game play to conserve mana to avoid this from happening.  I don't mind moving on to the next target as long as the preist has a chance to start to drink before you charge.  once I am drinking, i am getting mana and you can feel free to start whacking away.  The problem with charging before the preist can start to drink is that then he can't, because the whole group is then "in combat".   Then when the tank needs a heal, the keg is tapped out.    If the tank is paying attention to the play of everyone else, the tank can set a tempo.  Often in PUG's the tank doesn't realize the rest of the group is tapped out because they always have full health.  But the reason they have full health is because they are not controlling the crowd and the preist has spent all his mana keeping the mage and rouge alive while the tank was whipping around in bazerker stance.   If the tank is the main meat shield and everyone else is just crapping dps, ushually the preist will finish the battle with well over 50% and we can safely move on to the next engagement.   It is only when we get bad pulls and steal agro from the tank that the mana well taps out.  (or a big boss who takes forever and hits like a pimp on a slow night)  

I know this is very wordy, and by no means is a dictate as to how people should play a preist, but it is an overview of how I have tried to play.  I am always open to suggestion on how I can help out better, so please feel free to offer your imput.  If you have any concerns about my healing, which effects your game play, (maybe you feel I am slow to heal, or ignore you, or don't like use of shield) please let me know.   

Also, don't call out for healing.   I can see your health and know you need it.  If you don't get it, yelling for it will only annoyes your preist.  same thing for rezzing.  I first will rez others who can rez, preists/shaman/druids, then the rest.  

Hopefully this inside view of my mind helps you know what to expect when running with me.