Sometimes it's the predominant aura of the poem, sometimes it's the first line, or the last line, if it packs a punch.
Sometimes I just use the date, or date and where I was when I wrote it, or whatever was my major affect of the day.
Titles are not entirely useless. Sometimes the title sets the mood, or the context for the piece, which could easily be slanted another direction. Or else it puts focus on what you want to focus upon. Think of the titlt as the dipthong.(that's an awesome word, but I'm not sure I spelt it right)