Think on your themes
Trawl quotable lines from your book that touch on that theme, or on an important element
Check quotes/sayings etc for inspiration. Check lines in your book that encapsulate what it is about.
My book in my sig is kinda partially named for a song - but the Black is a very important thing (to my MC) If he fades to his black he is screwed...
This is a great start. This was what I did and there were quite a few possibilities from this exercise.
Just a thought. Sometimes I poke around on Amazon in categories similar to what I'm writing to see if that inspires something. Granted this isn't an exact science, but sometimes it helps.
This helps a lot, too. I did this as the second step.
Overall, it was still very difficult for me. The title was the hardest part and I often get asked how I came up with my title. After toying around with about five or six options during work in progress and then another five while the ms was with professional editors (all three of whom disliked all my title options), I thought about how I felt at my lowest.
The words "desperate" and "hopeful" popped into my mind. As I refer to a statue of the Goddess of Mercy in my book, I came up with this title. I asked the editor I trusted the most and she loved it. Everyone I spoke to liked it. So did my publisher.
Looking back at my list of 67 title options over the past two years, this was the only one that just sailed into my head. The rest were created after much thought and some agony. While I thought some were good, even if the editors disliked them, none captured the essence of my book quite like the one I now have.
I think some times it just happens. The title just comes to you, just like that.