Debian bug 342658 has illustrated some ways to not run an Open Source project.
Firstly, and by far most importantly, keep a detailed change log. An entry like "Support for Blah has been added" is essentially useless, because it doesn't give someone looking in a detailed view of what has happened. The only sane way to do this is to keep a proper ChangeLog following (something like) the GNU ChangeLog format. For an example of how to do this without fail, see the glibc ChangeLog. I can attest that single ChangeLog file has helped chase down more bugs than anything else, including revision histories. Other good examples are gcc, binutils and emacs. If I can't figure out why something changed from that, I can always go back to the person who did the change to find it, or cross reference it with mailing list postings.
Secondly, if you're writing a library then test cases to stop regressions are of fundamental importance. Again glibc gets this right, where fixes to major bugs must be accompanied by a test case. You should be aiming for correctness and portability, and there's just no way to confirm this unless you actually test your library.
Without these two basic facilities, anyone fresh coming into your code is left treading water without anything to hold on to. And since Open Source is all about getting as many eyes as possible on the code, you need to offer people something to grab on to to avoid drowning in your sea of code.