1-6: Stay in the lane. The clarity will help you stay as long as you like. With the first few skill ups being used for Dark Wind and Drain, you can harass better than anyone at this point. Make sure you use Dark Wind onto a minion near the other champ(s) and silence them as they walk towards you - usually they don't notice and are silenced when they come up to you... drain their damage away. Most of the time - they run, don't chase them. Most of this time should be spent on Dark Winding the minions (with some harassing) and getting some cash. At level 5, grab fear if you are laning against a rather ambitious opponent. Either you will get first blood or a kill.

7-11: This is where it's situational. Sometimes I will go "hunting" for someone to gank with my Crowstorm combined with my Flash (it's definitely a skill you want to have), or if I'm mid, I stay there gaining the kills. Try to get the Mana buff from the golem if you can but don't overstretch yourself. You should be a force to reckon with. Dark Wind, Fear, Drain = dead champ. If they still stay with you... Drain, Drain, Fear, Drain = any champ dead.
With fear and drain, you can drain half of their life away - easily. Be careful about those who like to stun or otherwise mess with your channeling like Xin or Cho'gath. Fearing them is key - otherwise use your dark wind in between drains to shut 'em up.

Why improved Flash in your masteries? Simple... more kills. Flash after a crowstorm and it's deadly. You can easily get into position to cut off the enemy champ from escaping. Be sure to get your timing right, nothing more embarrassing than to just show up without your storm on.

Why used Clarity, why not use teleport or exhaust? I find that I run out of mana more times than I have use for exhaust. Teleport can get you where you need to go in a hurry which equals more killing/leveling/money but truth of the matter is Clarity will keep you alive longer and able to lane as long as you want which equals more time in lane. If you get low on life from some arrant shots from the other champs, be sure to drain the cannon minion, he has more life.