The other solution is of course to lessen the amount of goods stored in your warehouse. There are several ways of doing that, three that I can remember:
- If you have available conductors (you often get that message when trying to unload trains, then the conductor will be busy) you can ship some to destination that requires it
- You can start a production using some goods - but if all the output slots in that factory are full, this will only be a temporary relief
- You can trash some goods. If you press a type of good in the warehouse, you have the option to trash one of ten units.