Do 'nt criticize the customer

Today I did very bad thing. I did criticize our customer for something, that I think they should not do. The tone I used may cause a problem in our working relationship. Rethinking about it, I recognized that there are many better options with the same effect than criticizing. People do care about tone and the way you express your message. With the customer, we should rather propose than criticize, use more informal and less formal way. With our staff, we should give more recommendation and less command.