Saw online someone recommended filling containers partway with packing peanuts, then topping off with soil. This way the containers are lighter, drain better, and you can actually move them around.
So far I'm rather fond of the idea, but we'll see what the first bad storm does regarding knocking things over. Until anything like that happens, they look stable.
We purchased 2 bags of packing peanuts from Staples, at around $7 per bag, and filled all 3 containers a little over halfway with the peanuts.
We followed this up with potting soil, packing it down lightly. A word of warning so you don't do what I did: If you water down the soil, it will pack itself down. If you plant immediately when the soil is at the top of the pot, once you water it down your plants are going to have a nice little 4 " rim protecting them. If you want your plant to be higher up, water down the soil and add more a couple of times.