While I admittedly haven't been playing as Soraka long, I have played healer and support rolls for years. I will attempt to provide a breakdown of each skill and my reasons for leveling(or not leveling) them.
Soraka is arguably the best champion when it comes to protection in LoL. Her ability to not only heal anyone anywhere on the map but to also restore mana to herself and allies make her the perfect lane buddy.
~ Abilities ~
Although a relatively low damage ability; Starcall's low CD, lowering of target's Magic Resist and the unlimited number of targets it can strike make it a good all-around spell for farming creeps. However because this particular build focuses on keeping other champions in-lane; Astral Blessing(up to level 2), Infuse and Wish should all have higher priority over Starcall.
Your main heal spell. Astral Blessing should be cast promptly and often. Because it raises armor by a good amount it can be advantageous to cast Blessing on your partner as they engage melee champs, even if they don't necessarily need the healing. The extra armor will protect them and keep them from needing another heal longer. Also don't forget to use Blessing to fill up health bars in between fights. While each level raises Astral Blessing's healing by 55 the extra mana cost can drain you fairly quickly, leaving you less infuses to use on allies and enemies; and with AP raising Blessing's healing at a 1:1 ratio, it can be more efficient to pump up your AP rather than leveling Astra Blessing. Although you may need to level up Blessing faster or slower depending on the circumstances of your battle, but if you have a good partner no more then level 1-2 should be necessary till after mid game.
Infuse is a great ability as it pulls double duty for Soraka, it not only keeps you and nearby champ's mana bars filled, it also does decent damage and silences baddies for several seconds at higher levels. This should be maxed as soon as possible. Only Wish should take priority over Infuse when leveling, and depending on the situation it can be leveled instead anyway. Unless your Partner is overly aggressive or a total mana guzzler you should be able to use every third cast on the offensive. In the lull between conflicts don't be afraid to use infuse to "top off" you or your partners mana, much more often than not it will be refreshed long before it is needed.
Soraka's Ultimate. Wish heals all 5 champions no matter where they are on the map. With high AP, a single maxed Wish can heal for over 5200 health!!! Although Wish has a shorter CD then other Ults. It is still in many ways simply a more powerful Astral Blessing with a much longer cooldown and larger mana cost. Wish should generally be upgraded whenever it can be, but can depending in the situation be held off for a quick level to Infuse or Blessing.
~ Item Order ~
Right out the gate get ~Mana Manipulator~, it will help both you and your partner keep you're mana up, meaning more abilities used by your lane buddy and more infuses can be used on enemy champs who may get greedy and push to far up. Next start getting items for the ~ Abyssal Scepter ~, it's 70 AP and 57 Magic Resist on top of your natural 18 will make you all but immune to casters early game as well as making Blessing and offensive Infuse more potent.
~Deathfire Grip~ can be substituted for the Scepter if your laneing against melee champs, the extra mana regen and CD Reduction will allow you to spam more heals and offensive Infuses. Depending on the situation you can get boots before or after the Scepter and even which shoe you choose is basically up to personal preference. I chose ~Boots of Swiftness~ not only to get away from enemies but to be able to effectively back up a partner being chased by 2-3 enemies without subjecting my team to a possible double kill if I fail. Now save up for the ~Innervating Locket~. I haven't paid a good amount of attention to exactly HOW much the Locket's passive helps other Champions but I've found that it's worth getting. Next I try to get the ~Glacial Shroud~, it's Extra Armor, CD Reduction and Mana are all good things for Soraka and it comes in one nice neat package. You may or may not need ~Frozen Shield~ at this point, but if you think you need the extra 60 armor or so, then go on and take it. While the shield's attack speed reduction is good, your gold could possibly be better spent elsewhere. ~Zhonya's Ring~ is basically icing on the cake as the game will most likely have either be won or lost at this point. Because Zhonya's Ring grants a whopping 120 more AP and it's unique passive raises your AP(including the AP gained by Zhonya's itself) by 25%, just Zhonya's and the Abyssal Scepter alone gives you almost 250 AP! That raises Wish's healing by 500 and Blessing's by 250.
~ Spells ~
My personal preference is Smite and Clarity. I like to take Smite for gold income and Clarity so I can use Infuse in a more offensive roll without depriving me and my lane partner of mana.
Improved Rally on the other hand can turn the tide of a team battle with it's extra damage, healing and AP. boosting Soraka's heals by 80 for the 20 seconds it's up. Careful placement of Rally determines it's effectiveness, too close to the front line and it will get destroyed, too far and the enemy just needs to pull back a few meters.
*End Outdated Information*
I don't like bringing Heal as I feel it's heal amount combined with it's CD makes it unreliable at best and a waste at worst, Mender's faith can make it work a little better, but in general I don't carry Heal at all. However as I said at the beginning, Spells really are personal preference, so take what you think will help you most.
~ Masteries ~
I've split my points between Offense and Utility, The extra AP, low tier CD and Magic Pen slots are very useful. One of the main reasons I've spec'ed into Offense is the Improved Rally slot(see Spells section) so if you decide not to take rally, there really isn't a reason to spec past the second tier of offensive mastery.
~ Runes ~
Real simple idea here, Raise AP, Mana Regen and Ability CD as much as possible.
~~ Stratagy ~~
~ Early Game ~
Your sole purpose at this stage in the game is to keep your partner in-lane as long as possible. If you have Smite use it as soon as it refreshes to gain gold, but your attention should be on: 1.getting the last kill on the melee minions and 2.healing/infusing your partner. At this point Infuse should be used only on allies, as it's damage and silence are too low and the mana gained is too valuable to justify offensive usage. That being said if you think you can get a kill, or your partner already has enough mana to finish the enemy champ, go ahead and cast it on the baddie.
~ Mid Game ~
By this time you should have the Abysmal Scepter and/or your Movement Shoes, Level 4-5 Infuse and lvl 1 Wish. If you brought along Clarity you can start using Infuse in a more offensive way, punishing melee champs who push up too far or silencing casters. Other than that just keep you and your partner's Mana and Health bars "topped off".
~ Late Game ~
By this time you should have 250 AP thanks to Zhoyna's Ring. With the bonus AP damage your Infuse should be the terror of the enemy's casters, inflicting around 400 damage and silencing them for 3 seconds. Heal your team, push any remaining towers and use starcall on minions and champions alike to help your casters take them out.
~~ Tips ~~
~ Using Boots of Mobility as your movement item can allow you to quickly cross lanes to cast a timely Astral Blessing, Which can in some cases mean the difference between a kill for your team and a kill for your enemy's.
~ When casting Starcall on minions don't forget to get in between the melee and caster minions so that Starcall hits all 6 of the minions rather than just the 3 in the front.
~ You can use Wish much more effectively if you know when to cast it. Don't forget to remind your teammates you have Wish at the ready.
~ Level 5 Starcall with 40% CD reduction can DECIMATE minions with it's 1.8 second refresh time...however it can also decimate your mana pool. Be careful of over-draining yourself when farming creeps and always keep your mana above 25% for a emergency casting of Wish.
~ A one-two cast of Wish + Astral Blessing can heal for over 1500 in the space of just a few seconds. That's enough to max out many champion's health from near death.