Assuming you have come to the raid fully prepared as any good shaman would do, you should know when to use all those goodies you brought along.  Yes, you can use everything at once, but I rather you ration out your consumables to cater to the events that will be taking place on raid night.

First thing I do recommend is applying the Superior/Brilliant mana oil to your weapon during the initial raid buff window.  The oil will last 60 minutes and will last through a random trash/boss wipe.  Make sure to oil your rod every 60 minutes for the benefit of all involved.

Next, for the rest of the items, you will need to evaluate your healing ability.  If you are new to raiding, your needs will be much different than those of a seasoned geared healer.  I will do my best at going over few situations here.

New Healer:

A new healer is probably equipped in mostly blue and greens with a purple or two.  They have about 1000 plus healing and MP5 while casting is maybe around 95-100 with a mana pool of 8k.

Trash Pulls: During the trash pulls I recommend eating fish sticks and downing a healing elixir.  They will give you a good plus healing, but if you do die, you loose everything.  This is the chance you will have to take as a beginning healer during a raid.  Also, use mana pots sparingly during trash pulls, if at all.  Only use them if you get a bad pull and mana is really, really low.

Boss Fights: This is where you will need to use everything you have to bump yourself to a safe level of plus healing and MP5 to be helpful during the fight.  As for mana pots, use them abound 70% to 75% to keep your mana pool up allowing you to possibly use another mana pot during the last 5% of the fight. 

Yes, you might die and lose all those nice buffs, but boss kills are why you bought the stuff in the first place.  You will find boss wipes can be expensive not only in repairs, but in lost consumables, but what are you going to do?

Seasoned Healer:    

You have run raiding for a while now and your gear is evidence of this.  You are now in 505 to 75% purples and you probably just got rid of your last green.  Your plus healing is around 1500, your MP5 is about 130 and your mana pool is about 8.5 to 9 k

Trash Pulls: For trash I would only recommend the fish sticks.  It will bump your healing up a little bit allowing you to handle most anything the trash can throw at you.  Plus if you die, not a huge deal.

Bosses: Like the newbie healer, use everything. You wills start to find healing becomes easier as you go, but you will still need all the bonuses you can get, especially to your MP5 as the fights become mana intensive.

The Healing Elite:

You have been raiding for months, you are all purple and there is nothing you can’t heal.  You are a god among trolls and you like to let everyone know it.  Your plus healing is tipping 2000, your mana pool is at 9 to 10k and your MP is 250 with water shield.  Your fear nothing and your tanks fear nothing.

Trash: Don’t use anything, heck bag the shield and the legs.  Besides the fact you can heal in your sleep and are probably not paying 100% attention to the pulls and it would be best that you don’t loose any of your invested cash on bad trash pulls that were obviously the hunter’s fault.

Bosses:  Meh, you don’t need anything here either.  You can solo heal half the instance so why would you need the help of any elixirs or fried fish.  Your uber and the zone knows it.  It is surprising that the bosses just don’t sit down and give up on seeing you enter the zone.   

Seriously, on trash, you don’t need to buff up and for bosses, just use everything no matter how elite you are.  There are times when anything can happen and you will need to make sure you are filled to the brim with healing bonuses.