Keywords are inclusive, so the more keywords you put in, the less lines you'll find.
You can specifically ask for a keyword to not exist by prefixing it with !.
For example, searching for "mouse !cheese milk" would find lines that contain both mouse and milk but not cheese.

[050127 04:08] 54a1b, Cam: curmudgionedness sounds about right.. and shove it up your undead arse.. *coughs and hacks and spits*

[050130 12:04] fad53, Kay: Hey Cam please e-mail me krazy-kay@snail-mail.net ! *hug hug*