1) Try to fulfill sawmill and workshop orders more frequently.
 
2) Run Flash Sales. The Flash Sale button appears from time to time on the top right of your screen when your storage is starting to fill up. Don't miss this offer!  
 
Flash Sale
 
3) You can also activate Samuel's recurring order (he will fulfill orders automatically). Upgrade this manager to increase the storage capacity and fulfill orders more frequently.
 
Samuel's recurring order