When Daniel Dufner Jr. left Sullivan & Cromwell as an associate in 2003, he took with him one of the firm’s big health care clients, Anthem Inc., to White & Case. Thirteen years later, Dufner is still at White & Case, as a partner. And he is still representing Anthem. Over the ensuing years, Dufner has helped advise Anthem on its $16.4 billion acquisition of WellPoint Health Networks Inc. in 2004; its $6.5 billion acquisition of WellChoice Inc. in 2005; and last year’s proposed $54 billion merger between Anthem and Cigna. (The merger has been approved by both company’s shareholders but still needs to win antitrust approval by the U.S. Department of Justice.)

So how do M&A lawyers like Dufner, who face stiff competition for multibillion-dollar deals, maintain these kinds of long-term relationships? According to top M&A attorneys and some their Fortune 500 clients, success lies in investing time to build and maintain personal and professional relationships with clients, having an in-depth understanding of the client’s business and how it fits within its industry, having your work evolve over time and putting together a solid, skilled team.