If you get mistletoe from the Christmas event and use it on one priest it will raise their spirit by 20. The thing is that the buff stacks from differant people, so if your in a raid and everyone uses the mistletoe on one person he will get an assload of spirit. I think we had a priest last night with 850+ spirit.
To get the mistletoe you have to /kiss the christmas people inside the inn, you can only do it once an hour