HRGuru

I am looking for suggestions on how to tactfully ask for and receive constructive feed-back about our internal interview process from interview candidates. Our agency has had very little turnover in the last 20 years and almost 80% of the staff is eligible for retirement, or have chosen to retire; others are openly toying with the idea. I believe having a process in place to receive feedback from candidates once the interview is complete will serve to improve/streamline the overall interview process as a whole. However, I am not 100% sure about what process I can or should use to collect the feedback /data from the interview candidates; any suggestions?