The easiest is to spray water on your palette while painting. I use a palette that has a wet sponge under a wet piece of paper. It comes with a tight fitting lid for overnight storage. A friend of mine uses an enameled tray, which she lines with wet paper towels. Use whatever meets your needs.
Acrylic paints are very tough, even tougher than oils.
Once dry, acrylics are permanent, flexible and water insoluble. If you wish, you can gently wash the surface of a dry acrylic painting with soap and water. Acrylics will not yellow with age, either. Another great quality of acrylics is that they can be used for a variety of projects where durability and flexibility are needed. You can apply these paints straight from the tub or jar, or thin them with water for watercolor or airbrush.