Today, the Electronic Frontier Foundation (full disclosure: I work there) launched a guide  offering key strategies for keeping a site “alive” amidst a denial of service attack. The guide, on EFF's website , is available in eight languages, including Arabic, Spanish, and Russian, and focuses on two main strategies for keeping one's site online: Backing up and mirroring content.
The guide also features videos created by the Tactical Technology Collective , such as this one explaining the nature of distributed denial of service (DDoS) attacks:
And here, EFF explains why this guide is important:
Denial of service (DoS) and distributed denial of service (DDoS) attacks are increasingly common phenomena, used by a variety of actors—from activists to governments—to temporarily or indefinitely prevent a site from functioning efficiently. Often, the attack saturates the target with server requests designed to flood its bandwidth, leaving the server unable to respond to legitimate traffic.
Though the owners of major sites often have the resources to fend off or even prevent such attacks, smaller sites—such as those belonging to small independent media or human rights organizations—are sometimes permanently disabled due to a lack of resources or knowledge.