One of the guides to follow, to enable and protect against HTTP DoS attacks in Apache, using mod_evasive:
mod_evasive is an evasive maneuvers module for Apache that provides evasive action in the event of an HTTP DoS attack or brute force attack. It is also designed to be a detection and network management tool, and can be easily configured to talk to ipchains, firewalls, routers, and more.