When training your dog it is of paramount importance that you pay close attention to what the dog is telling you. Often it is easy to get too caught up in training that you dont notice your dog is getting tired, bored or thirsty. I've fallen into this trap many times, especially when training is going well, and I keep thinking, 'I'll do it once more, then she can have a break" and before I know it she is unfocussed and not wanting to co-operate.
To try to avoid this, I've found that pacing myself is a must when training. Set yourself a target of training for a set number of minutes, and try to set a goal for this time. Be ambitious but not ridiculous, sometimes I set my bar too high and when the girls do not learn as fast as I expected, it can leave me feeling frustrated. Far more frequently, however, the girls far exceed my expectations, meaning that we all finish our short training session feeling happy because of our achievements.
It is also important, and easy to forget, that water should always be available for your dog when training, remember your dog is probably doing far more work than you, so they will likely get out of breath quicker. I've taken to bringing a training bag out with me whenever I train, with treats, clickers, toys, bowls and water inside, and whenever I return home I refill the water bottle out of habit, so I never go anywhere without water for the girls.