Standard or automatic transmission? The primary cause of run-on is an idle speed a little too fast, causing the engine to Diesel. If lowering the idle doesn't help, try leaving the transmission in gear, and letting the clutch out just as you turn the switch off. It's hard for the engine to run when it's locked up tight to a transmission with the driver's foot on the brake.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!