Give every device its own identity
Avoid shared default credentials. Establish a way to provision each device, replace its credentials when necessary and revoke access if it is lost or retired.
Plan for trustworthy updates
Decide how a device learns that an update is genuine, how interrupted updates recover and how long updates will be provided. An update channel that nobody owns is not a security plan.
Protect data while it travels and rests
Choose the data the device truly needs to collect. Protect communication, restrict service access and set a retention period that matches the purpose. Consider physical access to the device as well as network access.
Define safe offline behaviour
Ask what happens when the network fails, the clock is wrong or a sensor produces an impossible reading. The device should fail in a way that is visible and appropriate to its environment, rather than silently pretending everything is fine.