Ask the staff what they think they can improve on customer service, and make more sales, you can do this formally face to face. Feedback from customers can be positive or negative.
You can also do staff feedback, by asking your own employees how you think your doing and if there are any problems.
You can do this by questionnaires or a group meeting.
Usually an A4 sheet of paper with 5-6 simple questions for you to answer. This is so the company can get feedback and see what they are doing well on and what they need to improve on.
Example "John Lewis" they ask a question such as:
what can we do to imrove?
whats the most thing that you really like about john lewis?
A person is giving feedback on what they doing, and whether or not they're doing it right or not. Example if you go to a resturant and the waiter asks you whether you need anything else, that's what you call a good customer service they are keen to make sure that your alright. And if the waiter or staff don't really ask you that shows that they have a bad customer service, and they need to improve.
Number of complaints/compliments
If the company get plenty of complaints this implies that they're delivering bad customer service. They will be able to see if the complaints they are receiving are similar or different. If they are similar, it will be a sign that they are clearly doing something wrong and needs to be corrected immediately if possible.
Compliments are the same except they are doing the right thing and should continue to keep doing it. example, virgin airways got a complain from a customer which was six pages long.