The higher authority close
A higher authority is a respected person known by your client who is willing to give third-party testimony. The higher authority will most often be a satisfied client -- possibly the one who referred you to the prospective future client.
Source: Sales consultant/trainer Tom Hopkins
To set this close up, choose your higher authority and discuss the situation with them. Tell him or her that you'll be meeting with "Jim Johnson" at 2:00 p.m. on Thursday, and ask if they might be available around 2:30 p.m. to take your telephone call in case you need his or her input. Always offer to refer other business back to your higher authority in exchange for their involvement or to return the favor.
When you make the call, simply make the connection, do a brief introduction, and then let your higher authority tell your prospective client how great your and your product/service are.
If your higher authority is unavailable to take your call, ask for a testimonial letter and permission to use his or her name.