For OSX, you can go to your user's $HOME
(probably /Users/yourname/) and, if it doesn't already exist, create an .npmrc
file (a file that npm uses for user configuration), and create a directory for your npm packages to be installed in (e.g., /Users/yourname/npm). In that .npmrc file, set "prefix" to your new npm directory, which will be where "globally" installed npm packages will be installed; these "global" packages will, obviously, be available only to your user account.
In .npmrc:
prefix=${HOME}/npm
Then run this command from the command line:
npm config ls -l
It should give output on both your own local configuration and the global npm configuration, and you should see your local prefix configuration reflected, probably near the top of the long list of output.
For security, I recommend this approach to configuring your user account's npm behavior over chown-ing your /usr/local
folders, which I've seen recommended elsewhere.