Instructions
Load your game of "Nintendogs" and select one of your dogs to interact with. It does not matter which of your dogs you choose.
Tap "Brush" from the options available. A new screen will load with your selected dog and a brush.
Brush your dog using your stylus until the game alerts you that your dog is looking beautiful. You will automatically be taken back to the select screen.
Tap "Brush" again using the same dog. This time, instead of using your stylus to brush your already-groomed dog, leave your game alone. You will gain one trainer point for every minute the game is turned on. This allows you to gain an unlimited amount of trainer points while you're doing other things, such as sleeping.