For example if you wish to create a MefExtensions directory for a main assembly to use the DirectoryCatalog to load in plugins at application startup then add this to your sub project's post build event
SET DestDir=MefExtensions
SET DestDir=$(SolutionDir)\$(SolutionName)\$(OutDir)%DestDir%\
IF NOT EXIST %DestDir%. (md %DestDir%.) ELSE erase /Q /F %DestDir%$(TargetFileName)
xcopy $(TargetPath) %DestDir% /C