You can't really. I'm in the gold farming and bot business and we are under constant DDOS with additional pile-ons when someone in China wants us offline for a bit. Thing is, our DDOS is only in the 5 to 15 gbps range. I saw that the attack on one of the mtgox servers peaked at 38 gbps. That is a lot of traffic to mitigate - I'm impressed that they manage to still provide a service.
All they can do is try to have redundant servers and tons of bandwidth.
38 gbps, that's incredible. We regularly deal with 8-10. We use dosarrest and it's like it's not even happening. Amazing service. Recommended to us by rackspace when they couldn't help us. Expensive, but if you're a regular target, worth it.