How to heal for a raid as a holy priest?

I have a holy priest that I have with about 5 k score gear and I've healed for more dungeons than I can count but not really for raids. I think I've tried about two or three raids through the weeklies and grouped up but the other healers seem to have endless supplies of mana, awesome gear and they can last through the entire thing. By the time we're done I'm always out of mana and healed barely over half what the others heal. Is it something I'm missing or doing wrong? What kind of things can I do to maximize heals and conserve as much mana as possible?