When you get layer shift, check your belt tension first. There are other contributing factors, but this is by far the biggest cause of layer shift. As your printer gets broken in the belts will tend to loosen up. Also, as the weather changes things might move around and loosen up too. Most belts on hobby printers are kept tight with a cam. Each one is different, but they all have the general idea that you tighten it by rotating the cam until the belt is tight.