The preferred mode, at least in the US, seems to be to invite doctors to deliver lectures. Another is to fund this or that foundation, and perhaps to fund the doctors' research. Do read NYTimes article by Gardiner Harris and Janet Roberts:
... [R]esearch shows that doctors who have close relationships with drug makers tend to prescribe more, newer and pricier drugs — whether or not they are in the best interests of patients.
Here's a specific case:
Dr. Donald Hunninghake served on a government-sponsored advisory panel that wrote guidelines for when people should get cholesterol-lowering pills. The panel’s 2004 recommendations that far more people get the drugs became controversial when it was revealed that eight of nine members had financial ties to drug makers. The full extent of those ties have never been revealed.
In the story, all the big recipients denied that the money affected them in any way at all. They were doing it (and receiving money for it, whatever 'it' is) without it biasing them in any way. Some former drug company insiders, on the other hand, are more candid:
... [F]ormer drug company sales representatives said they hired doctors as speakers mostly in hope of influencing that doctor’s prescribing habits.
“The vast majority of the time that we did any sort of paid relationship with a physician, they increased the use of our drug,” said Kathleen Slattery-Moschkau, a former sales representative for Bristol-Myers Squibb and Johnson & Johnson who left the industry in 2002. “I hate to say it out loud, but it all comes down to ways to manipulate the doctors.”
And, here's how another (former) drug company executive put it:
“If a doctor says that he got flown to Maui, stayed at the Four Seasons — and it didn’t influence him a bit? Please.”