Using Mr. Clean's Magic Eraser
You can use a stencil for lifting an exact shape from your watercolor painting. If you want to put a boat in the water or geese in the sky, it is easier to paint the large area first, then lift up the object. Then, you can then paint the object on clean paper.
The advantage of lifting over masking is that lifting leaves soft edges, while masking leaves hard edges.
Also, if you hate an area of your painting, you can lift it up and have a redo.