1. Buying a Pet Spontaneously

Why This Is a Mistake: That doggie in the window may be darling, but he might not be the right fit for your family or lifestyle. And a mismatch could lead to frustration and heartbreak.

How to Avoid It: Fully inform yourself before you bring home a pet. Every dog or cat has its own needs, and some of those needs are specific to the breed. Terriers tend to dig; Abyssinians explore and climb. If there’s a breed that interests you, read up on it (try the website of the American Kennel Club, at www.akc.org, or the Cat Fanciers Association, at www.cfainc.org), talk to owners, and get to know someone else’s Border collie or Persian.

That said, not every dog or cat is typical of its breed, so learn as much as you can about a potential pet. “At a shelter, ask about the pet’s history, health, and temperament,” says Stephanie Shain, a director at the Humane Society of the United States. When dealing with a breeder, you should be shown where the pet was raised and be allowed to meet his mother and father.

2. Skipping Obedience Training

Why This Is a Mistake: Bad habits, which often develop quickly, can be difficult to train out of a pet. So unless you have the know-how to school an animal, you need the help of a professional.

How to Avoid It: Even before a puppy starts formal training, you can teach him simple commands, such as sit and stay. A puppy can begin formal training at eight weeks (and ideally before 12 weeks), after he has had his shots. “Between the ages of 8 and 16 weeks, puppies readily absorb information about the world around them,” says Andrea Arden, a dog trainer and the author of Dog-Friendly Training (Hungry Minds, $19, www.amazon.com). To help a dog stick with good behaviors, every few years take him for a refresher course with a trainer. (Find one in your area at the website of the Association of Pet Dog Trainers, www.apdt.com.)