This is not an in-depth guide - I will post one if requested! Also, this guide is not for novice players. You need a fundamental grasp of LoL to use this guide effectively. You must know the basics of laning, map awareness, champion skills and playstyles, dodging/positioning and teamwork.

I am 70 games positive with this build, it works if you can play it. The players I group with frequently all insist I take mid or the solo lane because they can't do it better and they know they can rely on me to carry.

If you are after kills, don't bother reading this guide. Soraka is (obviously) not a ganking champ. But she is a major carry, a steady pusher and I can pretty much guaranty you will walk out with an assist for 90% of the kills your team gets.
Soraka plays much better 1v1 than 2v2 with an ally. In mid (unless you have played against me!) no one has seen a Soraka and will treat you like an easy target - their mistake! When I take Soraka mid, my opponent champ's tower always falls first and for the most part mine never even gets damaged. I also have penty of money to stack some AP items which let me carry my team through teamfight after push after teamfight.

I use the flat AP quints to give myself a boost early game and the AP/level marks, seals and glyphs. Soraka doesn't need magic pen. runes as Starcall debuffs enemy champs pretty well and you're not in it for kills anyways. You could substitute health/armor/magic resist runes if you prefer, for more survivability but you're pretty durable as is.

Improved Clarity
This is a must! You are now not only the health battery for your team, but also the mana battery.
Improved Teleport/Flash/Ghost
I prefer Teleport myself, but depending on your playstyle you might find Flash or Ghost more useful.
Presence of the Master
Another must! Lowering the cooldown on your Clarity even further will improve your durability and your teams'.
Archaic Knowledge
Great for making your Deathfire and Infuse silence that much more punishing.

The rest of the masteries are personal preference.

ALWAYS start with boots of speed and three health pots. After that, your next purchase should be a Deathfire Grasp. I like to keep an extra Lucky Pick in my inv. until I need to fill my last spot, as once laning phases end you don't have much opportunity to go farm (you should travel with the team). The Hourglass and the Abyssal can be switched in order, depending on whether you need more magic resistance or armor. I haven't decided if I really like the new Tenacity item or not, but I have been playing with it recently. Substitute a Rylai's or Rod of Ages if you're not a fan.

Starcall
This should be your first ability. Since you can wipe out a creep wave with 2-3 casts you can then get assertive, if not aggressive, with your enemy champ.
Infuse
This should be your second ability. Once you have wiped out the creep waves with Starcall (and hopefully pinged the enemy champ a few times) you can punish the enemy champ for getting close to you and your minions or deny them minion kills by trapping them behind their tower.
Astral Blessing
I don't normally take this ability until level 3. Use the health pots to keep yourself above 50% and you will be fine. Once you start ranking up Astral Blessing, make sure it's rank never exceeds your rank of Infuse (you will run into mana conservation issues if you rank Astral Blessing higher than Infuse).
Wish
Rank up Wish whenever you can, of course. Since the cooldown isn't bad, use it as soon as it's needed.

Clarity
An absolute must. As I mentioned before, you are the health AND mana battery for your team.
Teleport/Flash/Ghost
I prefer Teleport myself. I can recall in the middle of a push, wipe out a wave or 3 of creeps in or around our base (including super minions) and Teleport right back into the fight. Most effective if I recall when Wish is up so I can heal my team while defending the base. But substitute Ghost or Flash based on your playstyle.

No one expects a Soraka to go mid - use that to your advantage!
Level 1-6
Travel in the middle of the first wave of your creeps. Avoid moving anywhere that leaves you without a friendly creep between you and the enemy champ. When you are roughly in the center of the map, begin your Starcall barrage. Hit the enemy champ if you can. One to three casts should have wiped out the enemy creep wave and you will most likely have 5 of your own left to hide behind. Get aggressive and push the enemy champ to his/her turret and start pinging away at it. Do not focus on getting champ kills yourself, if you get your enemy low ask for a gank. Silence the enemy champ at will with Infuse, to keep them on their toes. You can easily deny an enemy champ creep kills by pinning him behind his turret with the threat of Starcall and silence.
Level 6-12
Keep pushing as you should have the enemy tower at 1000 life or less, if it's not already down. Join your teammates in their lanes as needed to push, and drop Wish whenever it's needed (bot/top lane is pushing or tower-diving to get kills). Remember to let your team know when you hit 6 so they can ask for a long distance heal - it's very surprising for enemy champs to be fighting a tank/DPS in top/bot lane at 30% life only to have Wish bring back 20-40% of their health from afar. Once again, you are not a ganker and you should not be going for enemy champ kills. Just kill your enemy tower and creeps and keep your teammates alive with Wish. Go to base to buy items once you can afford the boots upgrade and the Deathfire Grasp.
Level 12+
Team fighting! Here your teammates will start kissing your feet. Stay in the middle to back of your team, make sure that the initiator on your team is no more than 1.5x the range on Astral Blessings away. Keep moving. Heal the tank primarily, unless a squishy teammate is getting ganked from behind. Your squishy teammates should know to not overextend. Try to keep as many allied champs in your magic resistance aura as you can and remember to use your Deathfire at the beginning of the fight if any enemy champ looks your way, then follow with an Infuse silence and let your team gank them. You should be able to keep your whole team up through a team battle if you avoid the enemies CC measures, and with Clarity you will be able to keep your team moving forwards.

Soraka is great for 5v5 teamplay. She is super support no matter what teammates you have and those single-target and global heals are tide-turners all throughout the game. Also, since I rarely see people playing Soraka (which makes me so sad!) people have no idea what you are capable of mid or how to fight you. Putting Soraka mid will ensure that she can complete her item build in a timely fashion and the mid-late game heals will be that much more devastating for the enemy team. A happy Soraka will bring more to the team than if you replaced her with a DPS champ or even 2.
I hope this encourages people to bring Soraka on their team, as it's lonely being the only one out there! If you have any questions or comments I welcome them (just please, no flaming).