Archmages Savvy - 0.60AP per level
Cripple - Buffs Exhaust
Sorcery - 3% CD reduction
Menders's Faith - Increases Heal
Perseverance - 4% total health and mana regain
Good Hands - 10% reduced death time
Expanded Mind - 5% maximum mana
Meditation - Increases Mana regain
Quickness - Increased speed by 3%
Intelligence - 6% CD reduction
Presence Of The Master - 15% off summoner ability cooldowns
I get exhaust simply because I find it helps though the entire game and helps me escape more often then flash could/can. I also get heal for the simple fact I am a healer and using my ult along with heal is just insane healing over a couple seconds. Also early game when my ult is on cooldown heal is a nice replacement for my lane partner. Most of the other things I get should speak for themselves but other then the cooldowns reduction masteries you don't have to listen to me on how to get them.
Skills to increase when you can
Astral Blessing x4
The reason I get starfall first is so you can kill all creeps in two hits fast and you can actually level and get money though the whole game. I have done many version of Soraka before and all my games ended with me being at level 14-16 simply because I could not kill anything. So if you get starcall you will have enough money to fund the items and you will level faster then most of the other champions.
Boots of Swiftness
Aegis Of The Legion
My games normally end there if not before I even get the Aegis but if a game happens to last longer I will get Stark's Fervor (if no one else has) other then that I really don't know what else I would get.
Mana per level
If you get the CD mastery and a CD item in-game (the one I get) you are at 40% cd with one item. You may go higher with runes but I find it a waste and I personally like the mana,armor, and cd the item gives me.
This is what I go for on her because once you get her to 40% cd reduction you can use her ult every 36secs. This becomes a life saver later and on top of that it helps your team push and stay alive in smaller fights because you wont have to worry that you won't have it later in another fight.