We have recently released a tool called "strong-registry" that makes it easier to work with multiple npm registries.
The tools allows you to keep a list of named registry configurations, switch your npm client between these registry servers and promote (republish) packages from one registry to another.
Announcement blog posts:
FAQ:
How is this tool different from `npmrc`?
A:
npmrc switches full npm configurations.
strong-registry switches only registry-related settings and takes care of things like a unique package cache for each registry. It has also a nicer CLI, at least IMHO.
Have fun,
Miroslav