I like to use small over turned net pots (other pots with extra holes poked in also suffice, but not as well - or spare small orchid pots). Or, styro tucked into the center of the root mass.
To take up space in the bottom of pots, I use styro, marbles (fish tank/flower arrangement type), decorative rock, pieces of broken clay pots. I have found the marbles and decorative rocks at the dollar store
You can also throw small pieces of styro, chunks of lava rock, leca, into your potting mix.