1. Look at other titles in the same area.
With Adultery for Beginners, I had in mind Carol Clewlow's book A Woman's Guide to Adultery, which I thought was a brilliant title. I wanted something like that, though obviously my own. I played around with text book ideas, substituting adultery for maths, geography, whatever.
2. Write a list (it may be a very long list) of words you associate with the book.
This could be place names, character names, adjectives, themes, verbs, nouns... I knew what became Kissing Mr Wrong was about Lu's hunt for a mythical perfect man, so I was playing around with ideas about perfection and Mr Right eg Looking for Mr Right and so on. Then I turned it upside down - the book was really about her mistaken idea of who Mr Right was, and how she actually needed Mr Wrong.
3. Find a phrase or bit of dialogue in the book that seems to say it all.
In my second book, Oliver tells Anna as he's seducing her that "Nice girls do." The book is about nice girl Anna going off the rails, so it sort of fits. They do, and she does. BTW My working title for this was A Girl's Guide to Hedging and Ditching*, which was using the combination of no 1 and no 2, as Anna is a garden historian.
4. Ask other people to brainstorm ideas.
I'm always amazed by other people's take on things - if I ask a class for a list of adjectives I can bet that none of the ones I thought of will be mentioned. Book No 4 obviously needed an Italian theme, preferably mentioning Rome. I had the longest list of words but still couldn't find a title. At one point I collared a bunch of my son's friends and had an impromptu eight person title brainstorming session. In the end, my lovely friend Nancy came up with A Single to Go, which needed just a bit of tweaking to become A Single to Rome.
That's 4 ideas - any more out there?
*I loved this, but my editor didn't. Another Woman's Husband had the working title The Sex Lives of Hamsters, which she didn't like either.