Have you considered “single-point post-transaction” reviews? “At the end of each transaction ask individual business colleagues within your organization for one improvement suggestion. This should be one improvement you and your external lawyers can make to continue to improve the service that you provide.”
This technique comes from Ann Page and Richard Trapp, Managing External Legal Resources (ICSA 2007) at 81. Keep the question simple, easily and quickly answered by a thoughtful client, and to the point of continuous improvement (See my post of May 27, 2008: post mortems with 7 references.).