This is my Sona build that I've created since learning and playing Sona, not saying its ideal for everyone's play style but for the way I play its a perfect fit. Also, this should be pretty obvious, but this is strictly for 5's since Sona isn't a very good 3's champion. Enjoy

I realize that this is a very odd build to have for a support character, but from what I've experienced people on the opposing teams don't like Sona's.
The 3 points in Archmage's Savvy are a no brainer, you need ap to increase your heals.
The 3 points in Resistance can easily be swapped into Hardiness but its Resistance has worked out well for me.
The Utility side is pretty basic stuff, I get the gold/5 point because throughout my games I rarely exceed 50-75 cs.

Clairvoyance - To help spot MIA's, spot the opposing teams junglers, catching opponents out of position or by themselves, and to help prevent ganks.
Flash - Obvious, quick and easy way to escape from a sticky situation or for better positioning for ultimate and teamfights.

Marks - Pretty standard Magic Penetration, not much else useful for casters.
Seals - Sona is very mana dependent so I tend to stick with the flat mp5 seals for the early game benefits.
Glyphs - CDR to maximize heals with damage and speed boosting along with the added benefit of reducing the ultimate.
Quintessences - Straight AP, I find that since this build doesn't build into much AP the AP Quintessences balance it out nicely.

Pros
Very durable mid-late game.
Very little mana issues.
Ally teammates will love you.
Cons
Very mana dependent early game.
Farm is slow unless team is constantly ganking.
Nearly useless alone in a lane.

I start off with Meki Pendant and 2 mana pots since you can heal yourself once you hit level 2.
I then go straight into getting Tear of The Goddes to help with any mana issues I may run into while still in the laning phase.
Next I buy Ionian Boots of Lucidity for the added CDR.
From there I build my 2 gold/5 items, Heart of Gold and Philosopher's Stone to help make me a little more tanky, additionally help with mana, and to make money since I leave the cs to my lane partner.
Next I build the Soul Shroud to help me survive team fights/ganks, and for the CDR aura for my teammates.
After that I tend to get Banshee's Veil for the added health and passive.
By this time the game is usually close to over and I rarely get past this point but if for some reason the game hasn't ended I finish my Archangel's Staff.
If the game is still ongoing then depending on the composition of the other team I replace my gold/5 with either Thornmail Rod of Ages, Force of Nature or Guardian Angel.

This is obvious, but for this build, you should never be alone in a lane or off by yourself.
During most team fights it's important when to use your ultimate, doing this can turn the tide in a fight into your team's favor. Most times after the tank initiates and everyone is lined up is the best time to use your ultimate. If your tank is someone with an AOE I prefer to save Crescendo until after theirs has passed, for example Amumu, although if your initiator is someone like Nunu then it is best to Crescendo while he is casting his ultimate for maximum damage. Crescendo can also be used while retreating from a gank or a bad teamfight. Staying near the back of your team while running away and using Crescendo can give your teammates the extra second or 2 to get away, while you can also speed them up with Celerity.
Properly watching your passive can also help you turn tides during fights, escapes and initiations. Most of the time your going to want to have Valor queued up for the additional burst, but I also find that having a Power Chord of Celerity is a big help when chasing down a kill or running away.

That pretty much sums up my guide, if you have any question or comments please feel free to leave them or to leave me a message. Hope it was useful.