Hahahaha. There is no one answer to this. Sometimes I know the titles to things almost immediately, sometimes I have titles SITTING AROUND waiting for fics ('The Holes in the Ground' was one of these -- I found the poem and knew I wanted to build a fic around it, but I had to wait for the right idea to come along), sometimes they appear during the writing process ('Twelve Pater Nosters'), and sometimes I find myself on lyrics/poetry websites at 2am frantically skimming for something vaguely pretty.

My titles are usually obscure single words, or snippets from a quote or a poem or a song. Or a scientific concept. But I try to avoid that last because it is way too much fun :D

Hair-pulling happens, believe me. It took me FOREVER to find a title for the current Merlin AU, but I eventually found it by reading websites of quotes pertaining to travel. Themed quote sites are often really handy if you don't want to go totally obscure.