There is an article in the current Atlantic Monthly (sub req’d) by Stephen Budiansky titled “Truth Extraction”.  Subtitled “A classic text on interrogating enemy captives offers a counterintuitive lesson the the best way to get information”.

The salient quote:

“The successful interrogators all had one thing in common in the way they approached their subjects.  They were nice to them.”

[Bold mine].
Budiansky writes about a “small and fairly obscure private association of United States Marine Corps members”  called the Marine Corps Interrogator Translator Teams Association [MCITTA].  The quote above was written by Marine Major Sherwood F. Moran in 1943.  From the article:  

Part of why Sherwood Moran became such a legendary figure among military interrorgators was his cool disregard for what he termed the standard “hard-boiled” military attitude.  [edit]  Stripping a prisoner of his dignity, treating him as a still-dangerous threat, forcing him to stand at attention and flanking him with guards throughout his interrogation…invariably backfired.  It made the prisoner “so conscious of his present position and that he was a captured soldier vs. enemy intelligence” that it “played right into [the] hands” of those who were determined not to give away anything of military importance.

One more:

Since [Abu Grahib and Guantanamo] scandals broke, many old hands in the business have pointed out that abusing prisoners is not simply illegal and immoral; it is also remarkably ineffective.

At this point in the ongoing public debate over the abuse of people in U.S. custody, it is vitally important that the facts be applied to the rules.  We have been treated to public denials by this administration far too long.  Beginning with the “Gonzales Memo”, and through the most recent McClellan press conferences, the administration has backtracked, slipped sideways, and built a rather impressive hall of mirrors to deflect any criticism of their actions.

The mirrors are now crack’d.  To the body of evidence putting the truth before the public, including the Veteran Intelligence Officers for Sanity, we may now add the voices of experienced Marine interrogators.

Is there anybody in there?
Just nod if you can hear me.
Is there anyone home?

[Pink Floyd, Comfortably Numb]