Big fight = pre stoneskin and refresh protect
Make sure cleric stance is off when healing (I forget this sometimes when questing)
Wait till tank attacks and takes 10-20% damage.
At this point, I usually cast cure I + immediate regen (if I know dmg constant)
Once threat meter pulls up, sleep any red targets and targets not taking damage.
Kite kite kite kite kite (to tank if you can)

Shroud at about 30-40% mana depletion (level 38)
Don't heal stupid more than once. Yell at them.
If people break sleep, let them know that is your strongest/cheapest heal.
If they keep breaking, let them die. They need you, not you them.

Know that most of this info probably changes at level 50.

Learn how to heal-tank (read: tank as healer, not heal the tank), because chances are you will have to do it starting with Sunken Temple, and being able to do so makes you a hero.

Don't rez unless you know that person is worth the mana burn.