Backup an entire repository to one file using git bundle:

git bundle create BACKUPFILE.git --all --tags --remotes

Backup an entire local repository (with cache etc.) using rsync

rsync -hxPavilyzH --stats --delete .git/ USER@HOST:/PATH/TO/BACKUP.git/

Automatically clean the repository of unneccesary files using git gc

> du -chs .git
17M     .git
17M     total

> git gc
Counting objects: 12937, done.
Compressing objects: 100% (7859/7859), done.
Writing objects: 100% (12937/12937), done.
Total 12937 (delta 5145), reused 12515 (delta 4872)
Removing duplicate objects: 100% (256/256), done.

> du -chs .git
15M     .git
15M     total