Customer Retention Project

I was assigned to in-charge a six sigma project regarding the customer retention. My company is a servicing company (provide certification service to client, such as ISO 9001, ISO 14001, etc) but customer lost problem is quite annoying us especially the recent 6 months. To be honest, our company has a strong sale team but no one cares how to retain the old customer, their key focus is only knowing to get new orders.  My first sense of this problem is mainly due to the cost, service and consultant's influence, etc....

Who can kindly share the liked data and the ideas of  methodolgies employed to go thru DMAIC cycle? What kind of statistical tools are recommended to use in this project?

Awaiting for yr response....