Sona is still a very strong champion even though she has been nerfed a lot. This build is for a heal spam Sona. Keeping your team alive for as long as possible makes it really easy for your team to dominate all phases of the game, from the laning phase to the end game.

With the Talon patch Sona's heal has been changed to give a armor/magic resist buff when you heal someone. I use this to reduce incoming damage on my team. For example: Zilean's bomb, Karthus ult, etc.

Pros:
-Great Heals
-Ok damage
-Great Speed Buff
-AOE Stun Ultimate

Cons:
-Squishy
-Sometimes Focused
-Can be slow moving early game

I play Sona as a full on support champion. She is NOT AN AP CARRY no matter how badly you want her to be one. Although you can Kill steal like a boss it doesn't make you a carry. With an AOE stun and short cool down heals and speed buff, Sona is a really good support champion. If you play smart and stay in the back of the pack it is very easy to play 2 deaths, 1 death, and even 0 death games. While in the laning phase I find it easier to just farm on the tower instead of trying to tower push, this also prevents alot of ganking attempts. During the game once team fights begin to occur, you are pretty much going to be spamming your heal, speed buff, and damage move constantly.

Marks (Red) - Magic Penetration; you can chose anything you want, but Magic Penetration seems to work good, especially with helping your team with a little extra damage from your Q.

Seals (Yellow) - You really need mana regeneration if you want to stay in the field as long as you can. Staying around and healing your team is key to winning.

Glyphs (Blue) - You can go full Ability Power, or half cool down reduction and half Ability Power, its your choice. The cool down reduction lets you spam more spells, keeping your team alive for that much longer, keeping your team moving quicker, and adding that extra damage to help your team ace your opponents.

Quintessences - Ability Power is fine, but again its your choice.

-Start with a Meki Pendant and two mana pots at the beginning of the game.
-Build a Chalice of Harmony on your first recall and if you can grab a Boots of Speed go ahead and do it, or wards depending on how much gold you have and your situation in your lane.
-For your boots you will most likely always buy Mercury's Treads, if you don't need Mercury's Treads then you can buy the magic penetration ones or either of the two that give you faster run speed.
-Get either Soul Shroud (if you are getting focused alot) or Morello's Evil Tome.
-Depending on if you bought Soul Shroud or Morello's Evil Tome first, it is now time to buy the other.
-Rabadon's Deathcap.
-Finish off your build with a Zhonya's Hourglass for more AP.
-You can now buy any thing you see fit, at this point I start buying Elixers

Meki Pendant - You need the mana regeneration.
Chalice of Harmony - You need the mana regeneration.
Mercury's Treads - Crowd control reduction is really nice for getting away.
Morello's Evil Tome - Cool down reduction, ability power, some mana regeneration.
Sould Shroud - Cool down reduction and more health points.
Rabadon's Deathcap - A lot of a ability power.
Zhonya's Hourglass - Ability power with armor.

For your first summoner spell pick a support skill - Clairvoyance, etc.

For your second summoner spell pick an escape skill - Flash, Ghost, etc.

I use Clairvoyance and Flash for the most part, but its really your personal preference and team composition.

When the game starts, get your Q (damage) and use it before buying any items. After buying your starting items use it again. While running to your lane use it once more to have your passive ready (giving you some extra damage for initial bush control fight). Don't worry about the mana it will regenerate before you reach the tower. After you get your ultimate at six, max out your heal (W). After maxing heal, max Q (damage). Make sure you skill up your ultimate at 6, 11, and 16. Finish off your skills with E (speed buff).

1 (q), 2 (w), 3 (w), 4 (e), 5 (q), 6 (r), 7 (w), 8 (w), 9 (w), 10 (q), 11 (r), 12 (q), 13 (q), 14 (e), 15 (e), 16 (r), 17 (e), 18 (e)

After getting your Chalice of Harmony, it is extremely easy for Sona to stay in a lane and farm for as long as you can. You can even solo lanes if you stay on top of your tower and just last hit. I've done it many times and had it be successful, letting my lane partner (lets say an evelyn is my lane partner) go on a ganking spree. If you are good at last hitting and smart on using your mana the farming phase of the game can be one of the easiest parts of the game. Being mana efficient is really the key though, you can take on the other team's solo top (if they have a jungler of course) with just a meki pendant until you are level 18 (I know this from experience :P) if you just use your mana wisely and last hit effectively.

8/27/11 - added info about heal change
6/17/11 - added some score stats pictures.
6/16/11 - changed skill sequence some
4/11/11 - Changed some items for the upcoming mana cost buffs, reduced mana regen and added more ap/survivablity