Listen to others, but always trust yourself. Always.
Imagine that you sell product X. X here could be anything: to me, right now, X is Apollo, a piece of project management software that allows you to manage and organise your working and personal life. But that's just an example -- you could be selling cakes. After a while, and a lot of hard work, you become popular: people buy your cakes (and my software), and things are cruising along nicely. As you become more and more renowned, your customers start coming up with more specific requests.
