You truly do not want to use your ability to clear the lane unless you have to like when defending against multiple people, when the wave has crashed into your turret, or when your laner is in a different lane so you have to push hard. Your main source of damage is your autos so you can easily last hit or clear minion waves without using any mana. Reserve your mana only for fighting, saving, prepping, or stealing. When the minion waves are coming in a line before clashing with your wave, you want to use your W or beam. For big waves that are already piled up, you want to use your E or Lotus trap. Your Q is generally good for all purpose wave clearing. When You have extra shots left from your R, you can use it to clear lanes.