Always, always, always ask for feedback. Do it after every sale. Include it with your invoices, your product shipments, or your follow-up calls to clients.
Doing this will help you:
- Identify problems early — if something is broken, how will you know unless you ask?
- Give angry customers a place to vent — give them the chance to tell you before they tell the world.
- Meet your biggest fans — identify the people who love you, love what you do, and are likely to tell friends if you make it easy.
Bonus points for using the opportunity to also help happy customers introduce you to a friend. Below is a great example from the smart word of mouth marketers at PrintingForLess.com:

Point #4 – Create new fans.
Think of yourself in the position of your customer. If you were buying something from your company what could they do after the sale that would totally blow you away and give you something to talk to your friends about?