Good product managers interact with customers constantly.
You visit them. You have conversations with them. You often have a long string of email threads with them.
And when you have an idea you want to test, you should contact key customers to get feedback.
But which customers?
I’ve always had a few customers that I can contact “off the record.” Three (or more) that I can just call without going through any internal nonsense of clearing the call with sales and marketing and management and legal. Just a few people who can give me guidance, who I know won’t tweet it or blog about it or call my sales guy for a demo.
I don't understand it but it seems that most departments actively prevent product managers and product marketing managers from engaging directly with customers. WTH? You don't learn to drive a car by reading analyst research.
Keep track of clients who have contacted you and those you engage in any type of conversation. Then when the time comes that you need to chat with a buyer or user, you have a list of people you already know. Those who are inclined to help you as a personal favor.
You’re not trying to create a customer database here. You probably have a sales or marketing system for all your customer information already; you certainly don't want to duplicate that. But you should maintain a contact list of people you know personally; people who are willing to connect with you again.
Products solve problems for personas. Personas represent real people. Put those real people in your personal contact list and tag them with your persona names.
Then when you need to understand how a persona would use a capability or how a message would resonate, you have a ready-to-use list of people to contact.
I’ve found people are quite willing to give advice—as long as you’re not trying to sell them anything. Just make sure you can always reach them when you need them.
Keep your own list of customers. (And don’t tell anyone about it).