Setting the environment variable NODE_PATH to point to your global node_modules
folder.
In Windows 7 or higher the path is something like %AppData%\npm\node_modules
while in UNIX could be something like /home/sg/.npm_global/lib/node_modules/
but it depends on user configuration.
The command npm config get prefix
could help finding out which is the correct path.
In UNIX systems you can accomplish it with the following command:
export NODE_PATH=`npm config get prefix`/lib/node_modules/