Checkout often takes a minute or two and feels like a necessary evil. Make the most out of this highly personal touchpoint. Conduct 'sprint surveys', one or two simple questions that can be naturally integrated into a friendly back-and-forth. You'd be surprised how much there is to learn about your customers!
How did you find us/hear from us?
Did you find what you were looking for today?
Keep it short and simple and ask close-ended questions. Those are questions that can be answered with a simple 'yes/no' or short sentence.
Remember to document your customers' answers, turn them into actionable KPIs and evaluate your improvement over time.