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This champion is currently my main AP caster.
Ahri is a AP burst caster. Taking mid is the best choice for her. She darts around battle avoiding skill shots while dealing good area of effect damage.

Pros
-Good Base Damages
-Little Item Reliance Early to Mid Game
-Low Cooldowns
-One of the safer picks for mid lane
-Effectively 4 Blinks (If you take Flash)
Cons
-Squishy, and Requires Getting Close and Personal to deal Maximum Damage
-Hit or Miss Champion
-Not a very good late game when she doesn't get kills

Similar to Morgana's passive, however it only goes off every 9 hits with spells (max 3 hits with 1 spell), gives 35% Spell Vamp. Great sustain in laning.
Does Magic damage on the way out and True damage on the way in, great for farming, harassing, building charges for Essence Theft, and using Essence Theft .
Throws out 3 projectiles that prefers champions, lower cooldown and lower range.
Pulls enemies towards you while slowing them, in essence a taunt where they don't attack you.
Short range blink, used mostly to deal damage or to avoid skill shots. Can be used to get away to some extent.

In team fights use Orb of Deception to hit as many enemy team members as possible, and use Charm whilst chasing to prevent escapes. Fox-Fire can be used to great effect when used with Spirit Rush and running into their team and then dashing our again. Using Spirit Rush is a great way to land perfect skill shots or reposition to prevent yourself from taking massive damage.
Ranges
Auto Attack-550
Flash-400
Ignite-600
Orb of Deception-880
Fox-Fire-800
Charm-975
Spirit Rush-450 (Bolt Range-550)

Magic Penetration Marks will allow your abilities to do more damage, combines well with your Q which already has true damage (Other viable options are Flat Attack Damage)

Flat Mana Regeneration Seals will allow you to use your spells in lane without running out of mana instantly. (Other viable options are Scaling Mana Regeneration, Health per Level and Flat Armor)

Flat Magic Resist Glyphs allow you to take a little more damage than your opponents expect, giving you a slight edge when it comes to fighting. (Other viable options are Mana Regeneration, Scaling Magic Resist, Ability Power, and Magic Penetration)

Flat Magic Penetration gives greatest damage of all Quints for late game, Ahri's early game damage is not reliant on AP as her scalings are not the greatest. (Other viable options are Flat/Scaling Ability Power, Flat Health, Movespeed, and Flat Magic Resist)
__________________________________________________________________________________________Good All Purpose Page
-Flat Magic Penetration
-Flat Flat Armor
-Flat MR Glyphs
-MS Quints (Dodge skillshots, more roam and more kite)

Viable Summoner Spells
Great repositioning spell that allows for great baits, ganks and whatever else you need.
Increases overall damage output and shuts down those with massive healing.
Great for replacing Ignite, it reduces their damage move speed, and resists.
Grants global map presence, I would replace Ignite with this
A decent alternative to Flash or Ignite
Anti burst spell agaisnt extremely high burst mid laners, should not be needed as your Charm should be enough to reduce their damage.
Don't Take These Summoner Spells
Passive spell vamp, no need for heal!
With the great survivability is it really needed?
Ahri is not a jungler
Your ultimate should be enough to avoid all the CC
Take either Athenes or get Blue Buff
Gives nothing to your kit

NOTE:At any point in any build, you can replace Lich Bane with Iceborn Gauntlet to maximize kiting and outplay potential.

I found that farming with auto attacks is easier than a lot of other ability power carries. Farming with Orb of Deception is easy once it starts doing enough damage or you just wait until the minions are low, best case scenarios are when the enemy champions stand behind the minions so that you get farm and harass all in one skill! Fox-Fire is not a reliable way to farm as it chooses a random target. Charm...single target....need I say more? Spirit Rush shouldn't be used on minions!

Ahri is great at zoning people out of experience range due to her early game damage and range. Her sustain is also amazingly high if used properly, making her quite viable to play ranked with. Her mobility and crowd control spell ( Charm) make her great for kiting almost anyone.

1)Use Orb of Deception to farm and harass.
2)Passive spellvamp must be used with massive damage spells!
3)When using Spirit Rush, use it wisely to dodge skill shots. These can alter game play dramatically if you dodge all skill shots. (Kinda like Blink Stalkers in SC2)
4) Charm can be used to set up hard to land spells for you team.

1)Her ability power scaling may look like total sh*t to you, but her high base damages make up for it. (Also her total ability power scaling isn't that bad)
2) Spirit Rush can dash through walls, unlike Riven's dash (although Riven's Q can go through certain walls on the third cast.)( Riven's Broken Wings can now go through most walls on 3rd cast that Nidalee can go through with her Pounce)
3)Don't get cocky, it'll lead to your downfall
4)Wards are the biggest game changers.

12/15/11-Started Making Guide, Finished and Published Guide
3/4/12-Changed item build, rune build, more scores, changed summoner spells section.
7/29/12-Mastery Changes, Item Changes, Skill Sequence
9/19/13-"Revamp" of most of the page, updated build to my current build. (Wards still save lives)