In a query, the first thing your future agent sees is the title. Think about it:
Your title needs to do a lot of heavy lifting. Make it strong.
(Yes, in theory the agent sees your name before the title, but unless your name is Nora Roberts or James Patterson--both of whom I assume have agents already--you're not really going to make an impression with your name. You could make a lousy impression with your name, but it's harder to make a positive impression.)
Your title, however, is the first place your book gets to show off, and it needs to be awesome. It needs to fit the story. It needs to convey genre. It needs to be intriguing.
Titles aren't like naming your baby. Titles are marketing tools. That's all. And in some ways, the title is the last thing the author gets to say about the story.
I'm a lousy titler. I know this, and it was only confirmed for me after an agent wrote me a long email requesting pages but asking for a new title please along with a bunch of suggestions as to how I could go about this. She didn't know this was the fourth title the story had already gone through. She also didn't know I'd see it two days later when she posted a cleaned-up version of that letter as a blog post. (Minus my name, which as we've already said is nonmemorable.)
Two of my novels have come right down to the last minute where the cover artist couldn't proceed because she kind of sort of needed the title before she could design the cover. That's Olympic-grade lousiness. (And we're not even going to talk about the face that two of my children didn't have names for the first 24 hours of their lives. "Honey, she's bringing the birth certificate paperwork. We really need to decide.")
I'm a lousy titler, and therefore you can assume I'm a pro by now at picking out ineffective titles.
1) Does the title fit with your genre? Occasionally you can make a title work across genres, but that's for later in the game, when you're an established crime writer and want to throw in a fantasy-esque title for flavor. Right now, reserve your fantasy titles for your fantasy novels.
2) Can this title work for half the books in your genre? If it's "To Love Again" or "Magical Lineage," try again. You need something specific enough that no one else's story truly fits your title.
2A) Does Amazon already have five pages of novels using exactly this title? This especially happens when someone uses a cliche or a quote as their title. Your title needs to stand out.
3) Is your title incomprehensible? I hate asking this, but sometimes in the heat of the moment, we latch onto a tiny element of the story; it becomes outsize in importance, and it makes perfect sense after you've read the book. Unfortunately, everyone else is seeing the title before reading the book, and the title gives us enough of a "huh?" feeling that we don't then read the book. I've seen this happen a lot in critique groups, where I'm obligated to read the story, and generally someone will tactfully raise the idea that perhaps the title needs an adjustment.
Keep in mind that the first thing an editorial board does is decide whether to change your title, so unless it's spot-on, you may not keep it. But that doesn't mean you should avoid doing the work.
1) Go to Amazon and look at the top hundred titles in your subgenre (free and paid.) Read the titles and nothing more. If there's a series name, look at that too. Just get a flavor for how the books indicate their genre in rough strokes.
2) As you edit and re-read your novel, look for a key phrase that encapsulates the through-line of your story. This is my favorite way of finding a title, although it doesn't always work.
3) Write down twenty ideas, good and bad. In fact, make sure you include plenty of bad ideas just to get the juices flowing because sometimes the fear of "getting it wrong" means we freeze up on our creativity. Instead, do what Gavin DeBecker suggests: make one of the qualifications for success that you have to be wrong more often than you're right. Once you've got that, you can brainstorm properly. Make sure to laugh at yourself.
And as a corollary: these are just for you, so go crazy. Try that twenty-word title. Use just your main character's first name and call the book John. Title it in French even though you don't know French. Pull out Roget's Thesaurus and derive alternate versions of ho-hum titles. Make puns. Make lots and lots of puns.
4) Draw up a list of themes underpinning the book and see if any of those resonate with the titles you've already played with.
5) Call your friend who always has awesome titles and sob into the phone for twenty minutes, hoping she'll say, "Well what if you turned the title backward and called it Half Missing?"
It's only a few words, or maybe even only one word, but the title carries the first burden of selling your work. Ensure it's a good one.