For Priests (Both Inquis and Saint bound) this will be the main guide for these forums. Feel free to add your thoughts and your own edits.
Gear: Blue/Rare gear till lvl 40/max level. +9 gear for the most part, +10-12 if you can manage. If you only want to invest +9 for those peices that will give you the most bang for your buck you'll want your Torso, Pants, and Hat in that order for Health and Mana. For damage do both your Shield and Wand.
Once you hit the lvl cap you'll want Epic class gear. Enhancing to +9 is best, but +6 is enough for Manticore.
Increasing Damage Your magic damage is increased by your INT, your hidden ablilities on your accessories, and weapons. Getting a magic dmg hidden is in most cases 2nd best. A Light Damage hidden is THE BEST for PURE damage. Light dmg +8% VS. Magic dmg +5.8% (duh)
Increasing your Healing To increase your Healing Aura, increase your max hp. The numerical amount is subject to the individual (I believe) healing 9% of your/their max hp. To increase your Healing Relic percentage heal increase your magic damage (found in hidden abilities, by increasing your INT, and enhancing your weapons).