This is my first guide so please be nice and don´t comment my spelling mistakes, etc. cause I´m not an english guy! ;)

First I have to say:
Kennen isn´t as easy as many peops think he is!
You have to know how and when to use your Q spell to hit them and to dominate the lane!
Also I gank after getting my ulti and dominating the lane.

Easy to tell you how you have to specc:
Important is to start with your Q spell cause you need to hit the enemy without getting too close.

You have to choose which spell you wanna max first or if you max all spells.
I play on maxing my W spell first to get CC on the lane (e.g. minions and champs, ganking, etc.)

To the runes:

Marks:
I use Magic Penetration marks.
Why?
It´s easy to tell you:
Kennen is a strange early champ and you haven´t enough time to buy Void Staff or your boots as early as you need ´em.

Seal:
Why do I use Armor seals on an AP Champ?
The answer: You need defense to resist the tower and enemy Champs (Normal hits and AD mid champs) ´till you get Rylais!

Glyph:
You know why I use Ability Power Glyphs, don´t you?
You need as much AP as you can to start the game with your skill shot ( Q spell ) and damage your enemy!

My masteries are easy to explain.
I play mid and an off mage, what I have to say?
22/5/3

22 Points on Offense to get AP and 5 Points on Defense to stay as long as possible on lane and 3 Points on Utility to get +10 Energy

You can choose how you like to play, I prefer to play with ignite and flash.
The reason?
You can ignite your enemy if he´s near to be finished and getting your masterie buff.
I use flash to escape from dangerous situations or in combination with my E spell to hunt enemy champs and finish them!

I use these Items cause you need Spellvamp on low level to survive ganks!
Your boots depending on the enemy team (off, deff, teaming, solo fighting, jungler, etc.) and Sorcerer´s Shoes are the best to pick.

The last 2 Items depending on your situation, if you need more defense you pick Force of Nature and / or Thronmail.
If you need more AP you pick another Rabadon´s and Void staff.
...

Pros:
-Good CC on early game
-Well lane controll cause skillshot and stuns
-High skilled players can easily gank other lanes
-Teamfighter with great CC ulti and spells

Cons:
-low deff
-low life
-if stunned you haven´t any chance
-you´ll be focused in every teamfight

Team Work is the most important thing while playing Kennen!
Try to manage your ulti and your CC stun (E + W) to kill the enemy team fast!

Remember:
By hitting With your E spell you earn 40 energy needed for your W spell!!!

For me Kennen is the best champ ever!
No champ can top him (Ziggs isn´t as good as Kennen, too, cause he got no stun)!
May you like him more while using these tipps!

If you have any questions, write a comment please!