It's everything. The success of your engagement and the possibility of future revenue depends completely on that relationship. It's why "consulting 101" is all about making sure that you develop a good relationship with the client and keep them happy. As a consultant, it's your job to make sure they look good. Because your client is your "champion" in the firm.
Nurturing this relationship often requires investing time with the customer to show them they are important. It may involve taking the client out to dinner (if appropriate) or celebrating major milestones of their success. Another avenue to develop this relationship is to send them tokens of appreciation (if appropriate) on their birthdays, anniversaries, the birth of a child, or the achievement of a key milestone on a project. Building and maintaining the client relationship is as important, if not more, than doing a good job because it engenders loyalty from your client.
A friend of mine mentioned to me two weeks ago that his project was solely due to one particular Director at the customer who was his client. She had brought them in and had introduced them to other decision makers so that they could expand the services provided within the customer. She had recently been promoted to Vice President, which was not only a major step in her career, but one that gave his consulting firm a lot more access within this customer organization. His first thought was to send her a gift to congratulate her on her promotion and wish her all the best. All I could think about was what a simple, elegant, and yet brilliant move that was going to be because it conveyed the fact that they were clearly very happy for her and they were truly vested in her success beyond just consulting. Imagine if one of your vendors, partners, or consultants did that for you. How would you feel?
In today's modern age of communication, we are all enamored with the convenience of email and social media. While these are wonderful tools for communication, sometimes a personal touch is a lot more effective and important to develop a relationship. Wouldn't a hand written card on your birthday have a greater impact on you that an email or e-card? And in industries like consulting where relationships are critical and directly proportionate to your success, wouldn't it make sense to invest in something a little different that re-emphaizes that personal touch?