Your goal is to make it impossible for the enemy laner to play, it's easier said than done, but you should be buying control wards to prevent ganks and deny vision. And you should be harassing whenever you can, but don't get distracted and miss out on CS.

If the enemy is playing behind minnions you could stack rend on a minnion and as they walk behind it shoot your Pierce through the minnion and at the champion, then hit E to Rend.

If the enemy has decent damage/burst potential buy a second Doran's Blade on your first Back, and maybe a third if you really need it.