Whether you’re an online or traditional business, make it in your customer’s interest to give feedback. Start by explaining the benefits and how you’ll improve service for them in the future.
If you have a website, create a link that clicks through to, say, a Google Survey document. This tool is simple to use for you and your customer, meaning they’re more likely to engage and you’ll find analysing the data much easier too. To increase survey participants, you could offer an incentive for them to take the survey, such as being automatically entered into a prize draw.
Even simpler, always ask your customers how things went at the end of each transaction. You’ll learn a lot and they’ll know you care about their service.
Really, the only rule about collecting customer feedback is to make it as easy as possible for customers to provide it.