 Mirroring is an easy technique for cloning your content across the web. In case of a DDoS attack, a mirror can keep your content online so that it is still available. A mirrored site can exist even if the original is down. The whole content of your site can be mirrored and spread in different locations on the web, offering the same content at the same time. This will make life much harder for a potential attacker. It's one thing to attack a single service than to take down its many copies. You can view step-by-step instructions on how to set up a mirror here.