In a post over on Medium.com Barry vd. Heuvel shows you how to use a recently added feature of Composer, the ability to use local repositories, to install Magento extensions quickly and easily.

I’m a fan of using Composer (in- and outside Magento), so I like to use that option. This works great for free packages listed on Magento connect or Firegento Packages, because you can just require the packages and run composer update. [...] This is all great for public packages, which are download through the Firegento repository. But what about private packages? Ideally we could also use Composer for the packages we purchase. [...] In this blog I’d like to explain how to tackle these 2 problems, so you can keep using the Composer workflow.

He walks you through the two steps you'll need to set up the module so it can be installed via Composer: creating a mapping (package.xml) and the composer.json. For the first he recommends using the Magerun modman tool to help with this. Creating/updating thecomposer.json file to work with the extensions is relatively easy. He makes use of the "path repositories" functionality to points the package at the "extensions/" directory using wildcards in the path name to allow for inclusion of all extensions without having to list each one (see this PR). Finally, to help make the process a bit more clear, he walks through a full example using the Amasty module.