Ideally, you'd have a battery (or supercapacitor) that'd keep you alive long enough to allow you to detect the main power supply cut-off and perform a clean shutdown. In the absence of that, anything else is merely damage limitation.

That said, some suggestions:-

* Use different filesystems for data areas of the system (e.g. /data), and mount OS filesystems (e.g. /system) read-only. That should prevent OS components from being corrupted, unless the user is foolish enough to begin an OS upgrade before cutting the power.

* Mount data filesystems with synchronous writes enabled. This will impair performance, and on reduce the life of flash memory-hosted filesystems by increasing the total number of writes committed.

* Give the use an obvious "clean shutdown" option.

* If your device is battery powered, monitor the remaining battery capacity and shutdown at some empirically-determined "safe" minimum level (e.g. 5% remaining).