I used the Git OSX Installer.
I used the Node.js Mac Installer to install the current version of Node. It installed to /usr/local/bin.
NPM is bundled with Node. It installed to /usr/local/bin.
I considered nave, nvm, and n, and settled on n due to its simplicity.
When I tried to install n with npm install -g n, I ran into a permissions problem. I followed this advice about how you shouldn't use sudo to install packages and set my user account as the owner of /usr/local instead.
To set a default n-managed node version, simply add n <version> to your bashrc.