A) have enough servers so when one gets null routed, it's not a huge deal

B) make sure your servers don't fall over while getting full line rate of garbage incoming (this is not hard for reflection or synfloods, but is difficult if they're hitting real webpages, and very difficult if it includes a tls handshake)

C) bored ddos kiddies tend to ddos www only, so put your important things on other sub domains

D) hope you don't attract a dedicated attacker