Keep it on mains power, but not at 100% permanently if you can avoid it — a battery held full and warm swells, and a swollen battery in a cupboard is a fire in a cupboard. A smart plug on a timer costs very little.
Turn off automatic system updates on the handset. An overnight update that reboots the phone and lands on a lock screen is a gateway that is offline until somebody walks past it.
And watch the SIM, not just the phone. Prepaid credit runs out silently, and most operators disconnect a SIM that has had no outgoing activity for a few months. A monthly check of both is five minutes well spent.