You are in lane, farming minions and the enemy champion is across from you trying to farm also.

He is a squishy champion, go to "Situation A:".
He is a tanky champion, go to "Situation B:".

Situation A: Take him out of lane. Turn your poison on so there is a nice trail behind you when you initiate. Throw your W behind him (assuming you are already 3, if not, don't worry about it.) Next, burn your ghost, this will insure that you will catch him and won't be wasting mana and farm time. When you get close to him, use your fling to toss him behind you. As soon as he hits the ground, put your ignite on him. This will cause a lot of damage. Your fling + poison + ignite. Which can almost guarantee a kill on a squishy champion and put you will on your way of getting fed and snowballing.

Situation B: Poke. Run along the back side of the minions you are farming with your poison on. This will not only help farm minions, but it will also do damage to the enemy champion. A lot of people get really ballsy and try and come attack you. They will step in your poison and it will do a little bit of damage. Just repeat this until you finally get them to about half health, then go to "Situation A:"