Use a small amount of acrylic paint at a time. Since it dries out quickly, squirt out a dab of paint onto your palette right when you're ready to paint. Getting the paint out too early gives it time to dry out before you actually use it.
Purchase a palette that includes a sponge designed to keep your acrylic paint moist. Also, between uses, store your paint in a container with a very tight lid.
Spray your palette often with water. Start with a light mist so you don't over-saturate the paint and make it runny. If it gets too thin, add more paint and mix it up.
Make your own moistening palette for acrylic paint. Place a damp paper towel on top of a paper plate. Cover the paper towel with a sheet of wax paper. Put a small dab of acrylic paint directly on the wax paper with the paper towel under it, which helps keep it moist.
Cover your palette with a damp paper towel and aluminum foil between uses. Also, storing it in the refrigerator helps prevent the acrylic paint from drying out.