How can I prevent my systems and networks from being used for DoS/DDoS attacks right now?
Firewall any unneeded services, turn off any unneeded services, this will reduce the number of services that can be attacked. As far as actually preventing an attack from succeeding the best you can do is buy the most powerful servers you can afford, and tune the software (while under attack if necessary) to handle as many connections as possible. Reducing the timeouts on connections to services will decrease the effect of a flood somewhat, but legitimate connections may fail as well.
