Not that I was ever in need,
for I have learned how to be content
with whatever I have. 
I know how to live on almost nothing
or with everything. 
I have learned the secret of living
in every situation,
whether it is with a full stomach
or empty,
with plenty or little.                        

Philippians 4:11-12


The Apostle Paul wrote the book of Philippians, and some of his letters/books, while he occupied a prison cell.

Most of the New Testament was written by Paul :
Romans, 1st & 2nd Corinthians, Galatians, Ephesians, Philippians, Colossians, 1st & 2nd Thessalonians, 1st & 2nd Timothy, Titus, and Philemon. 

Paul was an apostle, imprisoned over a span of several years.  A man who had suffered shipwreck, beatings, and being stoned till near death. A man who kept his faith and wrote the book of Philippians while held as a prisoner. A man chained to a Roman guard 24 hrs a day, whose last book was 2nd Timothy.  A man who was eventually killed for being a Christian. Historians say the Apostle Paul was most likely beheaded.

It seems though that Paul knew contentment, even while not being in the ideal situation, and even while knowing death was a subtle shadow. 
How did he do it ? 
What kept the cold, stone walls from stealing his warmth ? 
 
Lets look at a few sentences that Paul wrote and discover what strengthened him to endure:

...but I focus on this one thing:  Forgetting the past and looking forward to what lies ahead. (Don't dwell on the past-keep eternity in your sight)

...But we are citizens of heaven, where the Lord Jesus Christ lives.  And we are eagerly waiting for him to return as our Savior. (Our home is in heaven)

...May God our Father and the Lord Jesus Christ give you grace and peace. (Peace given by God)

...Always be full of joy in the Lord.  I say it again - rejoice!  (Joy in knowing you are a child of God )

...Don't worry about anything; instead, pray about everything. (Humble yourself in prayer)

...Devote yourselves to prayer with an alert mind and a thankful heart. (Thankful, heartfelt prayer, attentive mind )

...Fix your thoughts on what is true, and honorable, and right, and pure, and lovely, and admirable. 
(Guard your thoughts) 

...Keep putting into practice all you learned and received from me - everything you heard from me and saw me doing.  Then the God of peace will be with you. 
(Become a doer of what you read in scripture)

...You are his heirs, and Gods promise to Abraham belongs to you.   (Salvation is a promise from God to His children)


And so lets remember verse 13, from Philippians 4 above that reads :

For I can do everything 

through Christ,

who gives me strength.


Your strength is not enough and you will tire out trying. Allow Him to work through you.