I did this primarily based on blog postings at http://scie.nti.st/2007/11/14/hosting-git-repositories-the-easy-and-secure-way, and also http://book.git-scm.com/4_setting_up_a_public_repository.html. See these posts to see what I did pretty much verbatim.
The big configuration difference is that I wanted the git repositories to reside on the elastic-block store, but be accessible in the usual way. So, after installing git and gitosis and adjusting my gitosis-admin repository, I moved all the repostories from /home/git/repositories to /mnt/ebs/git/repositories. A symlink in the /home/git/ directory links to the repositories on the elastic block store, which makes them accessible to the git user as they need to be.