In newer versions of Git (confirmed with v1.7.8) it is possible to set named pretty-print log formats using git config pretty.named_format. These can be set at a machine-wide, user or file level with the <file-option> argument.
To create a log format called jespers_favourite or the whole machine use --system
git config --system pretty.jespers_favourite "%h%x09%an%x09%ad%x09%s"
For single user use ‘–global’
git config --global pretty.jespers_favourite "%h%x09%an%x09%ad%x09%s"
Leaving the <file-option> argument blank will default to setting the config file of the current repository, .git/config unless defined otherwise.