you should be able to understand the customer wants and ensure that you map their requirements into reality; this might be simple; behind this you'll have to put lot of hardwork, establish your crediblity.
business like these, run on crediblity, word of mouth; making quick bugs will be luring in fields like these, but if you are looking for a longer innings and willing to make it a career, you'll have to establish your crediblity, fair transaction;
i guess you'll have your satisfied customers list growing, if you deliver right for what you commit.