Soda pop cans also work well. Turn them upside down so pop-top is on the bottom. Another suggestion which I find useful - cut a circle of the water-permeable landscape fabric (Typar is one brand name) just a little larger than the diameter of the pot at the level of the top of the cans. This prevents soil from sifting down between the cans, styrofoam peanuts, or whatever you might be using. One caveat - the less soil you have in a container the more poorly it will drain, regardless of what space-filling material is below it. Perched water table is the fancy name. A 20-inch tall pot filled half or two-thirds with soil will not drain as well as one the same size that is filled with soil. Think of a wet sponge standing on edge compared to a wet sponge laid down on its largest surface. Judy in New Jersey where the snow is melting. So of course it is supposed to drop down into the 20s Fahrenheit tonight.