1 Corinthians 9:27

But I keep under my body, and bring it into subjection: lest that by any means, when I have preached to others, I myself should be a castaway (KJV)

Then: Wright (p. 120) and Baker (129, 136) both describe Paul's service in terms of boxing because the vocabulary in this verse and in 9:26 come from the Corinthian world of boxing. The intent is clear. Paul does not want his work to bring others to know Christ, yet find that somehow in the end, he himself failed. Wright also discusses the support some have for the ideas of ascetism. Nothing here though concludes that the body and providing for its comforts is a bad thing. Wright summarizes that all Paul does is find ways for the body to glorify God as he looks forward to heaven and the resurrection. The warning about not being qualified to finish the race is picked up by Dunn and Rogerson (p. 1334). They discussed the verse as being a precursor also. Paul was indirectly warning Corinthians about the possibility of their leaving the knowledge of Christ and slipping back into idolatry.

Now: God (likewise Paul) so knows the deceitful hearts of men that warnings concerning pride will always be needed. Speaking in plain language Paul personalizes the possibility to his own life. As others listen, he hopes they can see that if it is true of him, it can be true of them.

Extra source: Cornerstone Biblical Commentary 1 Corinthians (William Baker), 2 Corinthians (Ralph Martin & Carl Toney), ed. by Philip Comfort. Carol Stream: Tyndale House Publishers, c2006.
Eerdmans commentary on the Bible By James D. G. Dunn, John William Rogerson on line at http://books.google.com/books?id=2Vo-11umIZQC&pg=PA1353&lpg=PA1353&dq=1+Corinthians+9+commentary&source=bl&ots=5QeQaYWrMS&sig=S5F3KcA_TvNMyywqRiimySoUYtc&hl=en&ei=QqHPSouICIWj8AaZxqyABA&sa=X&oi=book_result&ct=result&resnum=8&ved=0CBkQ6AEwBzgU#v=onepage&q=1%20Corinthians%209%20commentary&f=false

1 Corinthians 9:23

And this I do for the gospel's sake, that I might be partaker thereof with you (KJV)

Then: Paul takes the approach in order to have the Corinthians and others reading the letter join him in eternal salvation. Wright (p. 117) points back to verse 22 which carries forward to "for the gospel's sake." For the gospel Paul cares not for his rights or his freedoms. He will deny himself so that the message, the knowledge and presence of Christ, harvests souls for the kingdom.

Now: Some are critical of the approach. An example in today's world is found on the Answering Islam web site. Some Muslims say that "becoming all things" means adapting deceptive practices for the sake of spreading the salvation message. The on-site answer is that Paul wanted to display deep-felt empathy for those needing ministry. Not hypocritical, he needed to understand the specific group of not-yet Christians. Being prepared to answer such criticism is essential to today's global mission efforts.

Extra source: "I Corinthians 9 and the Charge of Christian Missionary Deception" on line at http://www.answering-islam.org/BibleCom/1cor9_19-23.html