You can add PPA's in a script and excecute the script when you need to re-install these PPA's (for instance because of re-installing your system).
Execute from command line:
chmod 775 install_ppa
and copy/paste your PPAs into the file. Rearrange your code to be efficient. Example:
sudo add-apt-repository ppa:tiheum/equinox
sudo add-apt-repository ppa:am-monkeyd/nautilus-elementary-ppa
sudo apt-get update
sudo apt-get install gtk2-engines-equinox faenza-icon-theme equinox-theme
sudo apt-get upgrade
sudo apt-get dist-upgrade
Put the file on a partition you do not format during installing or on a stick and you can execute the file thus adding these PPA's.
You can also add downloads with wget in it to install tar files etc etc.
Commands have not been tested