i try to never waste a lick of paint. any little bits leftover get dabbed on to blank atc stock that i always have at the ready.
i end up with really cool backgrounds for future atc's this way. i often use little round sponge dabbers which hold a lot of paint...i can usually paint a whole background with just the extra that has soaked into the sponge. i get these cool circles when i press the dabber down firmly. btw, the face inchies in the "laugh" atc is courtesy of my crafty pal, lisa m.
i also save all the scrap papers that i test paints, pastels, pencils and markers on, they make great backgrounds too...like this stripey oil pastelle on vellum behind the cherry blossom stamp. just tryin' to keep the operations green, folks! :-)