Timing your drops is essential. Try to get into a rhythm of when to release each box. When a box falls off center and land towards the right or left, place the next box towards the opposite direction. Consider the weight of the boxes, as the metal variety are heavier and have a different trajectory. The best place to tap to release a box is on the box which is already stacked. This will allow you to time the drops and keep your pile as even as possible.