Parking break a good habit
My wife put her van in park while it was running to talk to a repair man who just arrived at the house. Actually, she thought it was in park, but was still in reverse. The van door was almost taken off as the van backed up and clipped the repair guy's van. It then continued backwards until is ultimately hit my son's car, which thankfully kept it from rolling over a 5 foot high rock wall down into a swail in the woods next to my house. It was the first three car accident the investigating policeman had ever seen in a driveway.
In short, use the parking break.