It's all about statistical probability. In order to reduce the number of dents you get from a-holes, you have to reduce the number of times your car is vulnerable.
1) Where possible park at least 2-3 spaces away from the last car in the lot or in a low-traffic area (find out where the employees park for example).
2) Where possible park next to an island and place your car closest to the island leaving plenty of room for the vehicle next to you.
3) Avoid parking next to tall/wide vehicles, they will place dents higher on your car.
4) Avoid parking next to 2-door shitboxes, their doors swing out farther and their owners don't give a s*** about their car, or yours.
5) Avoid parking next to family sedans, kids don't care about your car and when they throw open the door it will surely hit your car.
6) Choose a minivan over a family sedan, kids can't hit your car with a sliding door...but they might put their greasy fingers on your windows as they check out your ride.
7) If you must park between two undesirable cars, place your car closer to the passenger door of one of them. Most people drive alone and you decrease your odds of getting a dent if you leave extra space for the driver of one car instead of equal space for the driver on one side and the potentially non-existant passenger on the other side.
8) Pull into a spot as far as possible because the farther out your ass is, the more likely someone will back into it or clip your bumper pulling into the spot next to you.
9) Back in where possible. When people see the clean front end of your ride they are less likely to barrel into the spot, plus the front corners of most cars are narrower and more curved than the rear.
I received 2 dings in my Protege5 after driving it for 5 years with these rules. One occured when I had to park in a narrow slanted spot in the city overnight. The other occured in the Christmas Tree Shop parking lot. I was very far away, but it got busy and some pack of old ladies parked next to my ride and the back passenger dented my rear door.
I haven't received any dings in my MS6 after a year and a half. Needless to say, I refuse to take my car to the Christmas Tree Shop and make my wife drive her car if she is going to drag me there.