Flat collars are commonly used in clicker training and other non-correction-based training, such as puppy kindergarten. They are also effective in training small dogs, however they tend to lift the dog off the ground when giving corrections while the dog is distracted or in high adrenal mode. They are typically made of nylon or leather, and fasten with a buckle or quick-release connection.
Remember that training is an ongoing process. You will never be completely finished. It is important to keep working on obedience training throughout the life of your dog. People who learn a language at a young age but stop speaking that language may forget much of it as they grow older. The same goes for your dog: use it or lose it. Running through even the most basic tricks and commands will help them stay fresh in your dog's mind. Plus, it's a great way to spend time with your dog.