meh, it's hard. First prerequisite to effective DDoS mitigation? you need to have enough capacity (in terms of upstream transit/peering ports) to absorb the whole attack. If not? it's over, you blackhole the target at your upstream's routers (and thus throw out the baby) and the attack ends, or you don't, and your service is dead for all customers.
That's why I don't advertise any sort of "DoS protection" - I know that attacks that are bigger and badder than my network are fairly common. This is also why I'm not going to take any promises of DoS mitigation from anyone who doesn't have a terrifyingly huge network seriously.
Now, once you have enough upstream port capacity to soak the attack, you then filter the good traffic from the bad. This is a whole 'nother can of "very hard" but it's easy compared to getting the capacity in the first place. Note, this filtering becomes /way/ easier if you have some idea of the sort of traffic you are expecting, but it's still difficult.
There are "clean pipes" services that claim to do this for you, with varying degrees of credibility. The thing is, CloudFair is one of the smaller companies to offer this. I was looking at the offering from level3, (in my mind, considering their network capacity, probably the most credible provider of such a service. Also, their service claimed to work with all traffic, not just http and the like, so it would work for me.) but it sounded like the price was somewhere along the lines of "give us 25% of your revenue, and we'll give you half the cleaned capacity you need." I mean, even the regular level 3 bandwidth is between one and two orders of magnitude more expensive than the cogent/he.net mix that is common in my industry, so uh, yeah. I didn't spend the requisite six months with the salesman to get the real price, but I think "more than I can afford" is a likely guess.
I mean, the idea here, usually, is that the network providing this service is large enough that it has a whole bunch of peering connections and can filter the incoming traffic fairly close to the source. Even if you've got hundreds of gigabits of capacity at one location, if it's all at that one location, it's very likely that something else is going to gum up the works between, say Austria and you. If you've got a giant, global network, though, your traffic from Austria goes to your POP in Austria, where you can filter the "bad" traffic (assuming you figured out how to do that.)
And really, you don't have to filter /all/ the attack traffic, just enough that the target isn't completely overwhelmed. Like spam-filtering or anything else, nothing is 100%.
And that's how most of the low-end hosting world feels about it. The upshot is that we throw out the baby; the small customer who gets hit repeatedly by large DDoS attacks, generally speaking, has to change to a different provider, 'cause they get kicked off. I mean, if you are paying someone $20/month, and your enemies take that service down hard? yeah, after the problem is fixed, you are very likely to need to find a new provider.