My neighbor taught me this trick after she purchased one of our Corten Steel planters. She uses empty plastic water bottles to fill up the planter about half way and then fills the planter up with dirt to the top.
This approach has several benefits:
1) The planter is not as heavy because half of it is filled with air that is inside the empty water bottles.
2) You can use less dirt to plant.
3) Plastic bottles at the bottom of the planter allow for excellent drainage.
4) Plastic bottles at the bottom of the planter promote air circulation and will help to keep the roots of your plant cool.
5) This is a great way to reuse the bottles which may otherwise end up in the trash.
Tip: make sure the water bottle caps are nice and tight on the bottles. Each bottle should act as an air bubble and keeping the caps tight helps ensure the bottles do not collapse under the weight of the dirt.