I'll start off by saying this isn't the only way to get good at Nintendogs. There are plenty of other paths to go down, some of them also quite successful, but this is just one of the options. What follows is a list of various tips that you can employ to help be the most successful with your game. By successful, I am referring to money, trainer points, and overall relationship with your dogs.
1. Visits to the Kennel to buy new dogs should be few and far between.
When building your Nintendogs family, you should not have too many dogs. This will help you save money, and also you will have more time to spend with each dog, therefore having more fun with the game. Having one to three dogs is a good idea; when you have four or more dogs, the time spent with each dog is dramatically decreased because, since you can only have up to three dogs at home at a time, you will be keeping dogs in the Dog Hotel. Even if you have a solid rotation, you will still be spending much less time with each dog.
Also, you want to wait a good while before you buy a new dog. I usually try to earn at least $10,000 and have "time spent" be at least three hours before I buy a new dog. This saves you money because you have more time to build up your wealth before you spend it buying your new dog.
2. Keep your old dog(s) in the dog hotel when you get a new dog.
This is another reason why having fewer dogs is beneficial: you have enough space in the Dog Hotel to keep them all in while you spend time with your newest puppy.
This is important because overall, you will get more money and trainer points, as well as time with your dog, if you interact with it individually. You will build up more stamina on walks, leading to more presents and more money when you sell the items, you will have more time to train them for competitions, they will know more tricks, and you will have more fun with your dogs as well. I usually keep my old dogs away until my new dog's "time spent" is at least two and a half or three hours.
3. Train for agility from the very beginning.
As soon as your dog has enough stamina to make it to the Gym on walks (this usually takes three to four walks, depending on your version of Nintendogs and therefore, the location of points on the map), you should begin agility training. Agility is the most profitable out of the three competitions for a number of reasons.
First, the Championship Class for agility is the most profitable, money wise. Winning the Championship for agility earns you $1000, whereas winning a disc competition will only earn you $600.
Also, Championship Class for agility is the easiest to stay into out of all three competitions. Obedience Trials may earn you $2000 for winning the Championship, but the competition itself is nearly impossible to win. Disc is easier, but overall, in agility it is very easy to win multiple times in a row. Agility dogs, once they make it to the top, will amass very high winning streaks, and it is hard to drop down, and easy to stay up.
These are only a few tips, but put them into action and they will be very useful.