If you would like to slow the drying of the acrylics on your palette there are a few options. Most acrylic painters develop a way of working that includes stopping every 5 or 10 minutes to mist the palette with a spray bottle of water. You can also use a stay-wet palette which has a wet sponge under a membrane of paper that keeps the paints moist from beneath. And you can save your paints for the next day if you use a sealed palette.