Clipping Masks

When painting an object on a transparent layer, try creating a new one above it, right-clicking the layer and selecting Create Clipping Mask. Now when you paint, brush strokes will only show up where there are pixels in the layer below. So if part of the lower layer is clear, your brushes won’t work there on the upper one. This is helpful when you want to try out some new colours in an area you’ve already painted, because turning off the layer visibility enables you to flip back and forth between the new and original image.