Dear …
Thank you for your valuable contributions to __(name of services/project/etc)__. Your involvement with this __(program/project)__ has been a real blessing to __(type of people/organization/etc.)__. There are countless people who have benefited from the __(compassion/dedication)__ and professionalism that you impart in your role as a __(type of volunteer)__.
It is evident that you really care about __(briefly describe the essence of the project)__. You take the time to understand each situation on its own merit and your willingness to __(briefly describe what he/she does)__ is commendable. In addition to your __(counseling/work/etc)__ with __(type of people)__, I also want to thank you for your help in expediting the process by being thorough and timely with all your reports. I am impressed with the care you take to communicate with __(type of person - i.e. coordinator)__ and ensuring that all your documentation is accurate and current.
Your long term dedication, commitment and outstanding efforts to __(Name of organization/name of project/program/etc.)__ are nothing short of exceptional. You have helped to ensure the vitality and success of this __(type of program)__ and I want to commend you on a job well done!
Sincerely,