1) Be diplomatic
2) Be friendly
3) Be patient - willing to spend as long as necessary in conversation.

People in general react well to these.

In my experience mark manley's advice is good. Use IDP first and produce your licence only if asked; good clean copies are a start however I've never had any problem with showing originals. The police have every right to ask to see your documents and verify that you're legal.

In many years of driving in much of the planet, I have only three times experienced incidents in which it was possible (probable) that a police check was hoping for a contribution (without any overt request), and in both occasions my willingness to do whatever the law required and my patience and tact resulted in my being permitted to drive away without any contribution being made. These were in Venezuela (5 minute chat), Colombia (45 minutes), and Paraguay (2 hours).

Two exceptions:

1) Northern Bolivia - police and other officials routinely, and overtly, request contributions, HOWEVER you don't have any problem if you refuse, politely.

2) Western Tanzania - a police patrol asked us if we could give them some fuel, since they were running out . We gave them some fuel, and consider this the same as if they had been any other traveller.

On a couple of occasions, both in Tanzania, we've been accused of infractions which we had not committed. Tactful righteous indignation worked and we continued on our way.