Prison life

Richard G. Fecteau and John T. Downey were CIA agents captured by the Chinese in 1952 and not released until 1971 and 1973 respectively. They survived their years of often harsh imprisonment well enough to live very full and useful lives on their release. They have never gone to the media about their time in China and even this CIA Journal article was written with minimal input from them. There's a short version from The Times here.

Their techniques for survival

Never Give Up Hope
Scale Down Expectations
Create a Routine
Get Physical
Keep a Secret Space for Yourself
Remember that a Brain Cannot be Washed
Care for Each Other
Find Humor Where You Can
Be Patient