The best way is to setup a subversion repository (e.g. if open source, you can use GoogleCode, if not, try
ProjectLocker: Free Subversion Hosting, Free Git Hosting, Free SVN Hosting, Free Trac Hosting, Software Quality On Demand ).
Otherwise, you could keep two separate copies of the project and ship updates back and forth using diff tools.
Unfortunately, OS X seems to lack a truly great free SVN client like TortoiseSVN for Windows, but there are a few free clients out there (SVNX, SmartSVN Foundation), and some relatively inexpensive ones.