I love gardening, but buying annuals and potting soil can add up when you hit the cash register. This year to save money on potting soil when planting in large containers, I started filling the empty space at the bottoms with old balls that my kids no longer play with. This worked out beautifully because I saved MoNeY on soil, my pots were much lighter, and I have fewer balls to pick up at the end of the day (shhh…don’t tell my kids)!
HaPpY GaRdEnInG
~Robyn (Jersey Girl)
The Suede Sofa Blog 
I put broken up polystyrene in the bottom of pots for the same reason – we get less polystyrene packaging these days, but still some and this keeps it out of landfill for a while.