We use a plastic cafeteria tray on which we place wet paper towels beneath a piece of tracing paper. Your paint will be mixed on top of the tracing paper, but because the wet paper towels are beneath it, the acrylic paint won’t dry on your palette. Over time, as the tracing paper fills up with mixed paint you will need to replace it with a fresh piece of tracing paper. Don’t throw away the wet paper towels, but keep them wet. It is very important that you don’t over saturate the paper towels with water.  Keep them damp, but not puddling with water. Pour off all excess water from your palette before placing the tracing paper on top. We do not want the acrylic paint on top to be thinned with any water coming up through the tracing paper. There are palette products at art stores with a sponge and sheets that all fit nicely into a tray with a lid. Those work for acrylics as well.

Here is a short tutorial on setting up your palette...

Here is another little video about the palette that keeps your paint from drying. When I discovered this years ago it changed everything for me!