A: Having run into this problem myself, and not liking continuously being forced to remix my paint, I first tried the various paintbox systems commercially available, but found them variously unsatisfactory. So I got myself one of those flat square polyethylene (“tupperware”) boxes with a snap-on lid (approximately 12″ × 12″ × 2″), then I made a tray with indentations sufficient to hold a couple of ounces of paint in each of sixteen divisions from silicone rubber (using a two-part mold-making compound).

I filled the bottom of the plastic tray with foam rubber about 1″ deep, which I wet with a dilute solution of chlorine bleach (to inhibit the growth of mold), pouring out the excess water, then placed my home-made tray on top of the foam. When the lid is in place, the paint will keep for weeks, and when you let it dry out, the residue peels out easily.

Of course there also are proprietary extenders for acrylic paints that retard the drying process—you can use them as well.