Basically, I looked around and tried a couple different builds that ended up being either too tanky or too squishy, and didn't like how they played. Pure AP Soraka can get some really big heals, but wont live long enough to use them in most games. With no AP though you don't get to take advantage of her decent AP scaling on the heals, and they end up rather weak late game, when you compare it to the sort of damage being dealt.

Pro:
Decent survival
Fairly strong healing
Counters Karthus ult very well (this can be said of Soraka in general of course)
Almost capped CDR to get the heal and mana restore/silence as often as possible.
Minimal farm required, lets your carry get the minions in laning phase

Con:
Not as much AP as some builds, you wont have any 1k heal ults
Still a little squishy, without some support you will die
Cannot threaten well, so will need a partner to protect a lane

I have MP marks but that's mostly because I couldn't find any others that I particularly like. Armor for the seals just makes sense to me, you don't really need mana regen, and that extra armor early on can help in lane. You can go AP or CDR runes, I personally like CDR per level, because some of her CDs are pretty long, and that extra bit can really help. I go with fort quints to avoid being spiked down early, and because Soraka has one of the lowest base HP in the game.

I looked at various builds and saw a lot of 0/21/9, but personally would recommend either 0/9/21 (or 3/6/21 as above), because while you lose a little health and defense, you gain much needed CDR, and get a 10s CV with only 40s CD. The fort quints help make up for the loss of hp there as well.

I think one of the most important things to do with Soraka is to get the gold generation going early. They pay for themselves, and in the case of HoG, you can later build into the randuins. Going tome into lucky pick gives you some early AP, and the HoG gives you some health for a bit of survival. You may want to consider switching the order for those depending on who you are laning with.

For late game, I put in the upgrade to dfg, but you can sell it to get something else (banshee veil for some extra MR/health perhaps), depending on how the game is going.

Boots of Lucidity, again you want CDR. Your abilities are all you have, so you want to be able to use them as often as possible. You will NEVER run out of mana, use your skills!

RoA: Health, Mana, AP. only thing it doesnt have is CDR, but you should be close to cap anyway after soul shroud, and will be capped if you get the DFG.

Soul Shroud: Good health, CDR, Mana regen. And an aura to give it to your team too.

Abysal Scepter: AP and MR for you, lower MR for enemies. What's not to love.

Rabadons: Most expensive item in the build, but it just gives you so much AP, and the build is somewhat light on AP until you have it. While this is okay since you are not trying to be a carry, you will want some to make use of the good scaling you get for your heals.



Items I didnt take:

Aegis of the Legion: This is an excellent item, and if nobody else on your team is going to get it then you should. It can replace the lucky pick. It is very strong for teamfights, and will give you more survivability as well.

Philosopher Stone: I've seen some folks using philosopher stones (in fact I played against one such Soraka today) but I feel that as Soraka you don't need hp and mp regen, since you get health and mana through your abilities. The stones also don't really build into anything useful, so I decided to avoid them for this build.

Mejai's: This is another valid option, but I would not recommend it for most games. You will get stacks via your assists, but they can come somewhat slow, and a single death will get rid of a lot of progress. Also, it just screams "focus me" in higher end play, and if you are dead you arent helping your team.

I personally like teleport, it makes it easier getting places, and while Soraka does have the global heal ult, until very late in the game it is going to be a fairly long CD and being able to teleport to your team (or back to a lane) is helpful, since she is a little bit slow.

Clairvoyance is just an amazing spell when you know how to use it right, and as I said above, with these masteries you get a 10s CV that has only 40s CD.

Other spells that work decently as well are fortify, flash, and ghost. If I were to swap spells for whatever reason, I'd probably drop TP and get Flash.

It is really a tossup if you go infuse or astral blessing first, and can depend on your team and lane composition. Always max the ult, the heal amount goes up, the CD goes down, there is nothing not to like about that.

About starcall.. I don't use it at all until fairly late in the game, sometimes I wait until I have maxed infuse and astral blessing to take it at all.

Do Not Farm. You don't need to. Let your lane partner get the last hits and build up his/her items, you will get plenty of gold from assists (through healing and the silence), and from your gold generation items. Eventually you can use starcall to clear minion waves and get a push going, but it really shouldn't be necessary as long as your team is on top of things, and its still better to let one of your carries get them if possible.

Basically, my goal with this build was to have enough survivability through healing and silence until you can get support from your team, without sacrificing so much AP that you can be ignored because you're abilities don't do enough to impact a fight. It probably isn't perfect yet (probably never will be), and I'll be happy to entertain any suggestions that the rest of you have.