After posting Dr. E.A.S. Sarma's warning about the potential for conflict of interest created by research funding by industry, let me follow it up with a link to Dan Ariely's description of drug reps' tactics in getting doctors to compromise on their ethics:
... One clever tactic that they used was to hire physicians to give a brief lecture to other physicians about a drug. Now, they really didn’t care what the audience took from the lecture, but were actually interested in what the act of giving the lecture did to the speaker himself. They found that after giving a short lecture about the benefits of a drug, the speaker would begin to believe his own words and soon prescribe accordingly. Psychological studies show that people quickly start believing what is coming out of their own mouths, even when they are paid to say it. This is a clear case of cognitive dissonance at play; doctors reason that if they are touting this drug, they must believe in it themselves — and so they change their beliefs to match up with their speech.
The reps employed other tricks like switching on and off various accents, personalities, political affiliations, and basically served as persuasion machines (they may have mentioned the word “chameleon”). They were great at putting doctors at ease, relating to them as similar working people who go deep-sea fishing or play baseball together as peers. They used these shared experiences to develop an understanding that the physicians write prescriptions for their “friends.” The physicians, of course, did not think that they were compromising their values when they were out playing with the drug reps.
That last sentence sums it up nicely: the trick is to play this game in which doctors become "willing victims." There's quite a bit of literature on such shady tactics (some of which could be quite unsubtle): see, for example, articles by Marcia Angell and Daniel Carlat. There have also been several news stories.