I have a well maintained KOS list and I generally let people add themselves to it before I attack them. There are several ways to get added.

Ganking or camping any of my friends or guild mates
Using terrain exploits to gank
Random sapping
Random sheeping
Fear bombing
Interferring when I'm killing someone on my KOS
Tough talking on the forums
Alts if I find out about them and I try to find out peoples alts when I add them
If a ganking raid takes me down and doesn't move along fast enough, added ;)

Generally I don't bother anyone unless they do one of the above.