You must be either misinformed about how to gear a shammy or your doing something very wrong.
I honestly can't recall the last time I went oom.
Self buffed I'm around 600mp5 while casting, you should not run out of mana unless your unlucky to have under geared healers & tanks and NO replenishment specs in your group.
Mp5 food and flask of mojo are a must as well.

Nekorion's comment is very odd. My healer is a resto shaman and I love him. I never go OOM. Shamans are by far the MOST mana efficient healers. With buffs, pots, and food, I can get over 1000mp5 and can spam chain heal non-stop though any raid in the game... Which is great for my numbers, but is extremely boring. In 5 mans, between ripride and earthshield I dont have to cast another heal (on a well geared tank). In a raid, riptide often gets wasted when another healer just tops the player off. Tidal force is useless as well geared shamans have a ton of crit as it is - I am at 45% with 25 man raid buffs. Heck, the only time I use mana tide is when another healer runs out of mana. Tidal waves was a lame attempt to get us to cast something other than chain heal, but our healing waves dont compare to a holy Paladins single target heals.

I have to agree with Ari, although i don't think i've ever seen my Mp5 get to 1000, i'm mostly 232-iLvl gear. the only time i get even close to oom is if the DPS totally suck and we're close to the enrage timer, or i realize i don't have water shield up anymore. i don't waste food buff on Mp5 food, i go for the standard fish feast or haste. i also found tidal forces to be a completely useless talent, i keep meaning to go respec and move that point into something a little more used/useful. if it gave me 60, 40, 20% Haste, or reduced cast time by 100, 60, 30% on next 3 healing spells with cast time of more than 1.0 sec THAT would be worth taking, but 60, 40, 20% Crit bonus on a "medium" CD is just not worth it really. it always seemed to me to be a much better mage talent.