DDOS attacks are almost unavoidable whatever your security though mate. You can try to monitor for smaller ones, but what would happen at times of huge demand, such as day one patches for games? That's the trouble with large scale attacks on large sites with the ability to create huge amounts of traffic. If somebody is prepared to go to the trouble of preparing such a large scale attack, how can you stop them?
The only effective defence is not to be a target in the first place.