Now that you have the customer in the service drive, set the expectation, you are in control. Let them know to hang out till your ready, not come right up to your desk. The when your ready greet them at the vehicle.... Take control of the situation. Be Positive, You Got This!   

Do your part, Get them written up, follow through on what you say you are going to do. Not following through will diminish all that you are trying to do train your customers. Set the expatiation and hold yourself accountable for what you promise the customer.