Preparation.  First, it helps if you are prepared.  You already understand the general need or issue that your business must address.  Establish your time constraints and budgetary limits; and be ready to discuss those with your business analyst.  If you are unsure of the size of the project, commission a smaller feasibility study or situation analysis to examine the general scope and solution parameters.  Draw up a contract that defines the work to be delivered, as well as the project parameters and constraints.  Also, compose a mutual nondisclosure agreement and sign it with your analyst.  Additionally, give your business analyst access to your staff members and their explicit cooperation.