I don't think there is a cookie cutter answer. It depends on how sophisticated the attack is.
Your isp might help (unless you have Rackshack, they just turn you off.
)
Sometimes you can just move to another server, as the dummies that launched it hardwired the ip address. Sometimes they can follow you.
Sometimes the attack is following a pattern that can be filtered in iptables or in routers.
You need a network expert.