With the new patch I decided to start levelling my priest again. After considering LFD and the new talents, I chose disc and have mapped out to level 31. After trying out the build in a few randoms I've found that there's no probelms at all with mana and I spend most of my time casting smite.
-Enlightenment +15% Int
-Meditation: 50% spirit regen in combat
Twin Disciplines 3: +6% damage/healing.
Mental Agility 3: 10% off mana for instant spells (PW:S, Renew)
Evangelism 2: Smite and Penance 20% more damage; 30% off Mana.
Archangel: 3% Total mana returned and +3/6/9/12/15% healing when needed.
Free point (Improved PW:S)
Atonement 2: Heal while Smiting.
Glyph of Power Word: Shield or Penance
Glyph of Divine Accuracy: 100% hit for Smite
Smite: Cast to 5 stacks and maintain for DPS mode.
-Archangel: Consume Evangelism stacks for mana, healing boost or because it's going to drop anyways.
Penance: Use on cooldown and Evangelism has 5+ seconds remaining.
PW:S+Renew: Absorb 332 Heal 66 HoT 440/12sec before spellpower