You can't go wrong with Ahri. She's a versatile mobility assassin champion who can melt almost every champion on the battlefield if played right. She has been my main since she first came out and It's rare that I lose a lane while playing her. I do not suggest her to newer players, or players who hate skill shots.

+ Mobility!
+ Very High AP Late Game
+ Amazing Poke
+ Single Target Initiation
+ Charm leaves your prey defenseless
+ Short Cooldowns

- Squishy
- Hard to Escape when stunlocked.

[*] Lich Bane

I'm not going to lie, people have harassed me for purchasing this item early game. But in the end, it's helped me in every situation, and you'll understand why. It's an extremely good poke item and it gives you a nice 5% extra movement speed. Who doesn't want that?

BUILD ORDER
[*] Dorans Ring x2
[*] Sorcerer's Boots (Alacrity later on)
[*] Lich Bane
[*] Will of the Ancients /[*] Morellonomicon
[*] Rabadon's Deathcap
[*] Zhonya's Hourglass
[*] Deathfire Grasp

Ahri is an AP Mobility Assassin Mid Lane Suggested Champion who dominates the lane and teamfights by luring her foes in with a Charm and melting them with her Orb and Fox Fire.
I like to play a harassment Ahri. As in, I poke. As much as I possibly can.
No, this does not necessarily mean spam your Q until they teleport back to base or retreat to their tower. This means poke them with your basic attack, and hit them with your Q every once in a while if you can.

Never, and I repeat, NEVER engage them without your Charm(E), Never Towerdive without your Ultimate, and never get Fox Fire over Orb of Deception!

Step one: Farm Minions.

Farming minions is like a second hobby (Your first hobby being a lane dominater!) to Ahri. Throw your Q and finish the job if any remain. However, a lot of people don't seem to put Ahri's Q to full potential. Wait for the minions to line up and start fighting your own minions, then aim your Q at the center Caster Minion and toss it. Your Q will almost always hit every minion. After your Q has been leveled enough, you will instantly slay those 3 caster minions and leave only the Melee Minions and Tank Minion left. Easy lane pushing!

Step Two: Roaming!...Or Not?

Whether you roam or not depends on a lot of things.
Does your ally need a gank?
Is your ally's turret about to be attacked?
Are there wards!?
Who didnt ward!?!?

And that's exactly what might not make you want to roam! So don't feel too ashamed of yourself if you don't roam and gank an ally's lane. You don't want to end up giving the enemy team a free kill, do you?

But if you are able to gank, and your lane is being held nice and steady or you've been pushing most of the game, then you should probably help out a fellow top laner or bottom laner and score some kills.

Step Three: Do Not Underestimate Your Passive!

Your passive counts as survivability. A lot of people just throw it out there and think "Hm, I gained some health...Cool!", but you need to put this to better use. The best way to do that is to hit as many minions as possible.
At full build you can heal up to at least half your hp bar! Sometimes MOST of your HP Bar! You could heal yourself completely if you're a little under half Hp if you hit enough minions. You're instantly back in the game just from ONE spell. Never underestimate it and always put it to the best of uses! Yay for survivability!

Step Four: Poke!

You're getting Sheen / Lich Bane for a reason. USE IT.
After you cast your combo, immediately basic attack them for maximum damage. Lich bane does more damage than you think it does. Not only that, it doesn't leave you completely defenseless. Early Game Sheen poking can easily win you the lane.

Step Five: Know How To Fight

While your Charm is an amazing initiation spell, it's a lot more than that. It grants you 20% extra damage against the charmed enemy.
TWENTY. PERCENT.
That's a lot, late game!
Melt that annoying Jinx with two spells! Charm her and Q her! It's that easy!
So lower your fists, pull out your orbs and get ready to blow some kisses!

Step Six: Team Fight Initiation

Yes, Ahri is an AMAZING initiator. You can easily dash into their team, pop your Zhonya's Hourglass and while your team is giving them the beating their mothers aught to have given them, you can dash out safely and start slinging your orbs around.
Hah!
While this is a good move, it's also very risky. You could get stunned by a player who has quick reaction and you'll just get melted on the spot. That's a fun way to get your team screaming at you in all chat, isn't it?
No. Pick off somebody and engage the fight that way.
The moment someone gets close, charm them and waste your spells on him. It doesn't matter if it's a tank, a bruiser, or their carries. You're going to do some seriously massive damage to somebody and their first instinct is going to be to run because they got singled out and their team can't do anything about it unless they engage while he/she is charmed. That's not a good way to start a fight for them either though, now is it?
Just try not to go after a tank or bruiser specifically. If it's all you can snag, then reel it in. Trust in your teammates to help keep their carries off of you while your spells get off of cooldown in the next few seconds.

That's really all there is to it, for me at least. If you have an questions just leave a comment and I'll do my best to answer you. Enjoy playing Ahri!

Check it out! 12/1 in a 28min game against a Yasuo. Not bad!
http://postimg.org/image/gqfzx1u8h/

More damage than Fizz in both AP >AND< AD? Easy!
http://postimg.org/image/6hy25472j/

Climbing up the ladder!
http://postimg.org/image/w7o8seypj/

Most magic damage on the board!
http://postimg.org/image/fzj9vh2u3/