Visual Studio Post Build Event - Copy to Relative Directory Location
Here is what you want to put in the project's Post-build event command line:
copy /Y "$(TargetDir)$(ProjectName).dll" "$(SolutionDir)lib\$(ProjectName).dll"
EDIT: Or if your target name is different than the Project Name.
copy /Y "$(TargetDir)$(TargetName).dll" "$(SolutionDir)lib\$(TargetName).dll"
You could try:
$(SolutionDir)..\..\
If none of the TargetDir or other macros point to the right place, use the ".." directory to go backwards up the folder hierarchy.
ie. Use $(SolutionDir)\..\..
to get your base directory.
For list of all macros, see here:
http://msdn.microsoft.com/en-us/library/c02as0cs.aspx