Shameless plug for a tool I developed in order to easily switch git identities based on projects: https://github.com/cquintana92/git-switch-user

After configuring the identities you just need to run

    $ git su Personal
    $ git su Work

And all the identity configuration (email, name, SSH key and optionally PGP key) will be set up into the repo's .git/config file.

Saved me a ton of time.