Make sure you have a hotkey for your water shield buff. Rebuff it after every blizzard and you will have almost unlimited mana. Also like the others mentioned, use healing totems and also the glyphs for healing totems and water shield(+30% more regen). I chain heal as soon as I see the blizzard animation and heal everyone to full. Always make sure your guys are full mana for the next blizzard or you might not live. Another thing is keep your laying down the AE on your paladin or whatever tank as he needs to keep the adds off the shaman.