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If you're a paid developer on a project, then set guidelines early on about what the money can and cannot buy. This does not mean you need to post twice a day to the mailing lists reiterating your noble and incorruptible nature. It merely means that you should be on the lookout for opportunities to defuse the tensions that could be created by money. You don't need to start out assuming that the tensions are there; you do need to demonstrate an awareness that they have the potential to arise.
A perfect example of this came up in the Subversion project. Subversion was started in 2000 by CollabNet (http://www.collab.net/), which has been the project's primary funder since its inception, paying the salaries of several developers (disclaimer: I'm one of them). Soon after the project began, we hired another developer, Mike Pilato, to join the effort. By then, coding had already started. Although Subversion was still very much in the early stages, it already had a development community with a set of basic ground rules.