Your goal, as a professional adviser, is to always help the client to make the right decision (for them). Rather than "selling them," your goal can be to help them and have them decide to hire you when that is the right decision for them to make.
To help them make the right decision, you can strive to become their discussion partner as they think through the issues you know something about. Your "sales process" is to:
Listen to understand the situation as the they see it including their view of the problem definition, the costs of failure to the organization and to them personally, and the history of addressing the problem
Use insight to help them understand their problem/opportunity by using your professional knowledge, your business knowledge, and your skills in thinking about and communicating about issues in organized ways.
Identify the value of success. Help them articulate the financial, strategic, and emotional benefits of successfully addressing the issue.
Help them make the decision. Help them define the criteria for deciding among options, outline the various options, and assess the risks of all of the options (including the options to "do nothing" and "do it themselves"). One option they will be considering by this point is hiring you to help them.
Then you let the client decide, and have strategies in mind for what you will do if the client makes a choice (including a choice to not hire you) that you deeply believe is the wrong choice.