how to respond to possible attacks

[Robin Becker]

Sorry if this is too off topic, but I would like to find out what to do 
when you suspect a possible dos attack on your system. I know there are 
many experienced sysadmins here.
Although my system (freebsd 6.0/apache 2.0.x) did in fact hold up, what 
steps should I be taking? The originating ip doesn't seem to be reverse 
mappable.
-- 
Robin Becker