I learned this from Claudine Hellmuth in her first book, Collage Discovery.
First, make sure that your surface is well-sealed. I use gloss medium (any brand will do; I use Liquitex). Then apply a good layer of Vaseline wherever you do not want the paint to adhere; err on the generous side of both quantity and placement--it's easier to go back and do this technique a second time than to try and scrape off dried paint. This is the voice of experience talking. Next apply a coat of liquid paint over the Vaseline (heavy bodied paints are difficult to spread without getting Vaseline all mixed into your brush). After the paint is dry wipe the Vaseline/paint off the canvas with a paper towel. As a final step, wash the surface with soapy water to get the greasy residue off. If you are working on watercolor paper, as I was with the postcard, just use a baby wipe to wipe the surface.