You will need a plastic container that seals tight, wax paper from the kitchen, a paper towel and water.
Fold a paper towel to sit snugly at the bottom of your container. I fold the two sides to meet in the middle and then turn it upside down so that the smooth side faces up.
Next, I cut some wax paper to size. I choose a strong wax paper, since I don't want the paper to tear in the middle of a project. I tuck the ends of the paper in so that it doesn't lift easily when I work on it with my brush or palette knife.
I now wet the paper towel with water. I let all the excess water run off, without pressing any water out. I then lay the wax paper on top of the wet paper towel.
Any air bubbles that get trapped under the wax paper or paper towel creates a vulnerable spot that will tear easily. For this reason, I make sure to push all the trapped air out, rolling over it with a sponge roller.
Your palette is now ready to receive your paint. The paint will stay dry for hours (even days, depending on the weather). When breaking, simply cover the palette with the lid and you can come back the next day and continue painting. No more hassles trying to match the colour you mixed yesterday that has since dried out.
Looking at my lid, you will see an added benefit. The lid itself becomes an extension to your palette, providing a smooth, flat surface on which to mix colours. Once satisfied with the result, I simply lift the colour off the lid using a palette knife and transfer it to the wax paper surface where it will stay wet longer.
Links to Products:
Acrylic paint: http://aprettytalent.com/products/iris-core-acrylic-paint