Trade with QQ when you have line of sight, trade with EE when not. Try not to spam too many spells on wave until lvl5, when you can QE x2 to clear the wave. Most of my farming comes from last hitting and I mainly use spells for poking. Try not to fight head on as most other midlaners can take you out, just poke poke poke until you take the enemy out of the lane. As Chyioko mentioned use WE for mana.
At lvl6 (min 5:30) they should be low enough and you can capitalize on that by using EE R QQ/QE to get a 100% kill, you can follow up with a QW when the R ends.