I wrote a post-commit hook for just this purpose. The hook itself is simple; just add a file named post-commit to your .git/hooks/ directory with the following contents:
git push my_remote
The post-commit file should be executable. Also make sure that you add a suitable remote repository with the name my_remote for this this hook to work.
I also made a symlink named post-merge that points to post-commit. This is optional. If you do this you’ll auto-sync after merges as well.
UPDATE: If you want to ensure that your server, and your mirror do not get out of sync, and ensure that all branches are also backed up, your post-commit hook can use:
git push my_remote -f --mirror