One of the most important things that crossover trainers (trainers who are changing from a correction-based approach to a positive one) learn is that training a dog is a fun process of helping their dogs enjoy learning. Breaking the training process into small enough steps for success allows the dog and trainer to enjoy the journey because they are both being challenged while still achieving high success.

Progress means moving towards a goal in small enough steps that that each particular dog needs. Often those steps are much smaller than the trainer expects. Asking for more than a dog can do sets the dog up to fail. Asking for less than he can do (or doing too many repetitions of the same thing) sets him up for boredom. Finding the 'sweet spot' where the dog is steadily making progress with the behavior and is having fun learning is the real training challenge.

Making the Process an Enjoyable One

A good rule of thumb is to set up the environment and choose a specific training session objective (criterion) so that your dog can achieve at least 50% success rate on what you are asking him to do. That means for every 10 repetitions, he gets at least 5 correct and is marked and rewarded for each. That level is the minimum level that most dogs need to continue trying and the behavior is a fun puzzle to figure out. Practice at that specific objective until your dog can get at least 80% (8 out of 10 repetitions) correct before changing the objective to make it just a little more challenging. Just like a good computer game, we need to increase the challenge to keep the dog's interest and joy in playing with us.

Observe and Note the Pattern of Success & Failure

The trick here is to observe the pattern of success. If your dog gets 4 or more in a row correct, he probably is starting to understand what you are asking and you can ask for something a little more challenging. If he gets three in a row correct then makes a mistake, then he needs more practice at that level. Alternatively, if he gets something wrong twice in a row in a training session, then it is up to you to make something easier so he can succeed the third time. That success will keep him motivated to keep trying despite the failure.

Changing the Objective on the Fly

If your dog did get two in a row wrong, you will need to immediately change something about what you are asking to make it easier.  This "in the moment" evaluation is called "formative evaluation" in education circles. Applying formative evaluation during training sessions is a key skill that allows you to focus on the training process rather than the end product.

With an enjoyable (just the right challenge level) process, a good product is more likely. The desired final behavior can then be achieved.
Ultimately we are looking for progress not perfection!

Footnote:
As trainers, we need to remember that while we can get behaviors to be reliable, they will never be "perfect" since dogs, like us, are variable in their behavior no matter how well trained they or we are. That is another reason we focus on progress, not perfection.