Hint: Free Gifts and Toys
While you can purchase new toys for your pup to play with, you can actually get some goodies for free if you know where to look. When you take your Nintendog on a walk, outline a path that goes by the blue "?" boxes marked on the map. Your puppy is likely to run off screen at those points and come back with a present for you!
Your puppy will also randomly run to get presents for you if you leave the game on the "Home" screen for long. If left unattended, your Nintendog will leave the apartment and stay off screen. Simply tap on the center button (the circular one) to whistle and call back your pup who should be carrying a new present.

Hint: Gift Locator
To find out if garbage or a present is approaching during one of your walks, look at the top screen. There should be the little dog symbol on the path that you drew. If that dog symbol stops and your dog is still walking, this indicates that either garbage or a present is coming up on the floor. If it is a present, tug your dog to a stop and your dog will notice it. It will pick it up and bring it over to you. It is easier to do this when your dog is running.

Hint: Beat The System
Prior to attending a contest, save the game. While attempting the trial, you may opt to turn off the Nintendo DS and reload your save (before the contest) to retry without docking your daily contest limit.

Hint: Easy Trainer Points
Navigate to the screen where you wash your puppy. Instead of actually brushing the puppy, leave the brush over your puppy (in the center of your dog). For each real-time minute the brush stays there sits there, you should gain one Trainer Point.

Hint: Walk Till You Drop
Before making the last stop in the park, change your dog's collar to auto-save your game. Turn off the DS and cold boot the game. This will enable you to save any items you found during the walk and restarts your walk at the same time, allowing you to farm the park for items.