Ensuring the availability of your systems would be to keep them up and running and make sure they can handle lots of web traffic. I would keep the systems up to date and make sure that we have backups in place if something were to happen. I would make sure that the backups are tested as well so they can actually be restored properly. I would also make sure that we have DoS attack prevention in place. These things would be firewalls, vpn, and anti-spam. Firewalls and vpn will help keep people out that are not supposed to have access and anti-spam will keep the servers from getting overloaded by constant pinging.