I bought a set of Painter’s Pyramids to help me while I painted my real life kitchen cabinets, but they are now in my mini tool box. :D
They are plastic pyramids with holes on the sides so you can use your fingertips to reposition them easily. They are also hollow on one side, so you can stack them for storing. A package of 10 is roughly $5, though it would make more sense for a pack of 12 since you typically use them in fours for better stability.
They now have a version with holes so you can attach them to your work surface with screws, and one of the packages I bought has this feature. For my cabinet doors, I did find the doors slid around on them a bit, but the pyramids stayed put even without the screws. I think it was the nature of the semi-gloss surface that made the doors slide around. When I used them to paint the kit walls with craft paint, there was no sliding. :]
You paint one side of your board, wait until it’s relatively dry and then flip it over onto the tips of the pyramids. This means your painted surface won’t stick to the work surface. Even waxed paper and foil can stick to a tacky painted surface and pull paint away in spot. If the finish is marred at all while using the pyramids, it will be only at the points where the tips hold the board — a relatively easy touch-up.
You can work so much more quickly than you would be able to without them. And, painting both sides of the boards at the same time keeps warping to a minimum. The pyramids allow both sides to dry evenly.