Understanding your dog's body language will make you a more efficient trainer. Be sure to take some time to learn what your dog's trying to communicate through his or her behaviors.
One of the easiest places to begin is with your trainer. Ask them to point out what different positions and stances mean. If you're in a puppy socialization class, then your trainer probably will or would be willing to explain different play styles and what the different dogs in class are saying to one another.
You could also ask your trainer for book recommendations for better understanding your dog. Other dog people in your life may also have suggestions.
There are numerous infographics illustrating various dog movements and signs. I've collected a few on Pinterest for my easy reference.
Even if you can only spend a few minutes looking over one of the infographics depicting dog body language, you will have a better idea of what your dog is saying. You'll be able to more easily see the signs that your dog is getting tired or bored or frustrated.
Recognizing these signs early will help you either change up your training game or recognize that you've reached the end of that day's session. Either way, you'll be strengthening your bond with your dog.