It’s true—sometimes pilots forget to close their flight plan altogether. A long time ago ATC placed my little Tiger in an NDB hold in weather with 20-mile legs, obviously a delaying tactic, awaiting the ILS. Then a Short 360 commuter (the “Winnebago with wings”) showed up, also seeking the ILS. On inquiry, ATC told us that an aircraft had flown the approach and not closed its flight plan. Sure enough, the airport manager found the airplane at its tiedown, pilot long gone. Claiming minimum fuel, we landed first with only half an hour of fuel left in the tanks.
Failing to close your flight plan can be more than inconvenient. In our case it could have been deadly. Search and rescue kicks in 30 minutes after your ETA, which is sound reason to update your ETE if it changes that much or more. And they can bill you for false dispatches. So, many of us use a little reminder as a last resort in case we forget to cancel or close our flight plan, be it IFR or VFR. Does your airport have a sign on the exit gate reminding you to close your flight plan?
I know one pilot, who when out practicing locally, will leave a paper copy of his flight plan on the seat of his car. Get in the car; see the flight plan as a reminder to close if he hasn’t already. The memory trigger can be anything from the subtlety of turning your wrist watch around or pulling your handheld radio out of your flight bag and putting it on the floor. Others can be more overt like sending yourself a text message or setting an alarm on your phone. Whatever it takes, close your flight plan. Sooner is better, hopefully before the reminder kicks in.