If you're managing programmers on an open source project, keep them there long enough that they acquire both technical and political expertise — a couple of years, at a minimum. Of course, no project, whether open or closed-source, benefits from swapping programmers in and out too often. The need for a newcomer to learn the ropes each time would be a deterrent in any environment. But the penalty is even stronger in open source projects, because outgoing developers take with them not only their knowledge of the code, but also their status in the community and the human relationships they have made there.
The credibility a developer has accumulated cannot be transferred. To pick the most obvious example, an incoming developer can't inherit commit access from an outgoing one (see the section called “Money Can't Buy You Love” later in this chapter), so if the new developer doesn't already have commit access, he will have to submit patches until he does. But commit access is only the most easily quantifiable manifestation of lost influence. A long-time developer also knows all the old arguments that have been hashed and rehashed on the discussion lists. A new developer, having no memory of those conversations, may try to raise the topics again, leading to a loss of credibility for your organization; the others might wonder "Can't they remember anything?" A new developer will also have no political feel for the project's personalities, and will not be able to influence development directions as quickly or as smoothly as one who's been around a long time.
Train newcomers through a program of supervised engagement. The new developer should be in direct contact with the public development community from the very first day, starting off with bug fixes and cleanup tasks, so he can learn the codebase and acquire a reputation in the community, yet not spark any long and involved design discussions. All the while, one or more experienced developers should be available for questioning, and should be reading every post the newcomer makes to the development lists, even if they're in threads that the experienced developers normally wouldn't pay attention to. This will help the group spot potential rocks before the newcomer runs aground. Private, behind-the-scenes encouragement and pointers can also help a lot, especially if the newcomer is not accustomed to massively parallel peer review of his code.
At CollabNet, when we hired a new developer to work on Subversion, we would sit down together and pick some open bugs for the new person to cut his teeth on. We'd discuss the technical outlines of the solutions, and then assign at least one experienced developer to (publicly) review the patches that the new developer would (also publicly) post. We typically didn't even look at the patch before the main development list saw it, although we could if there were some reason to. The important thing is that the new developer goes through the process of public review, learning the codebase while simultaneously becoming accustomed to receiving critiques from complete strangers. But we also tried to coordinate the timing so that our own review came immediately after the posting of the patch. That way the first review the list sees is ours, which can help set the tone for the others' reviews. It also contributes to the idea that this new person is to be taken seriously: if others see that we're putting in the time to give detailed reviews, with thorough explanations and references into the archives where appropriate, they'll appreciate that a form of training is going on, and that it probably signifies a long-term investment. This can make them more positively disposed toward the developer, to the degree of spending a little extra time answering questions and reviewing patches themselves.