Religion is wonderfully weird. I'm not a very religious person, but I love all the tradition and oddness of ritual. I feel that the flavor of religious practice is sometimes missing from games and this is my attempt to fix that.
I envision every clerical domain as having a different style of prayer, and each domain comes with a bit of sacrifice; something the cleric commits to, which connects him to his deity. The religious act separates the Cleric class from the ordinary clergy, who don't do magic. This isn't meant as a mechanical change, really, just flavor.
A few I've come up with:
Air
Prayer: Air clerics climb a high place and meditate silently in the wind. They climb a tree, or up on a rock, or sit on a stool if they're trapped indoors (which they hate).
Sacrifice: Frequent fasting; air clerics tend to look pretty gaunt.
Animal
Prayer: Wear animal skins or horned headdresses, dance and caper while imitating animals, growl and howl, paw the earth, etc.
Sacrifice: Eat no cooked food.
Chaos
Prayer: Slip into an ecstatic trance, writhing and gibbering as chaos takes over.
Sacrifice: losing control sometimes, occasionally drifting into trance states at random.
Any ideas? I would love a brainstorming session on this.