I'm going to begin the with the disclaimer that I have never actually titled a novel in my life. However, while examining a friend's bookshelf the other day, I noticed that novel titles usually fall into a surprisingly small number of catagories, which should make coming up with them easier in future.
The name of the protagonist
Possibly the most obvious one, and the default document title for the works in progress of many aspiring authors. If you're feeling pretentious, you can always take the character's name and turn it into a fancy noun: The Odyssey and The Aeneid being about the only examples I can think of, so it's probably a bit of an outdated idea. If you want to be slightly more creative, you could go for a description of the main character instead. Adeline Yen Mah's Chinese Cinderella is so named because the main character is Chinese, and has a hateful stepmother. Little Women because they're like women, but smaller. Les Miserables, basically because everybody is miserable.
The name of the antagonist
Like the protagonist's name, this can be a literal first name and surname, or a descriptive title. For a while, the only example I could think of was The Lord of the Rings, but Pride and Prejudice is also the name of the antagonist, when you think not just of characters, but of any obstacle which your protagonist has to overcome.
The name of an important plot device
The Golden Compass. It can be a physical plot device (such as Pullman's Golden Compass) or essentially anything which drives the plot (The Hunger Games, for example, although this could be considered setting. Which takes me to my next method.)
Where is your novel set? The Secret Garden? Or if you're feeling more descriptive, somewhere like Cassandra Clare's City of Glass? If that isn't working, also try considering when you novel is set - think George Orwell's 1984, for example.
A description of your plot
This one is a little more
difficult to make sound like it's a legitimate title, I think, and
possibly applies better to children's books. But imagine if
someone asked you, "What's your novel about?" and you answered in the
shortest way possible. Diane Duane's Wizards at War is, as I understand it (having not read the series), about wizards, at war. Your readers will know exactly what to expect, and anyone who likes reading about what you've written about will know just from the title that they ought to pick up your book.
A quote from a poem
Moving away from purely descriptive titles, this one is possibly a bit more work, since it requires you to find a relevent poem, and pick a meaningful line. But it will pay off, because quotes from old poetry make excellently profound-sounding titles. Libba Bray is a particular fan of this (think Great and Terrible Beauty, Sweet Far Thing, and Rebel Angels). To find relevant quotes - if you don't have the patience to read through poetry collections - try using sites like Wikiquote, searching the key themes or words of your novel, and seeing if any of the quotes you turn up leap out at you
An atmospheric word
Stephenie Meyer has said that in order to title Twilight (previously called Forks), her publishers sent her a list of atmospheric words, and she chose her favourite. It may be a little more difficult to make your word relevent, but it definitely has potential for a snappy title.
Read through the novel you're trying to title, and check if one of your characters ever says anything particularly pertinent, or if a certain phrase crops up a lot. To Kill a Mockingbird, for example, revolves heavily around the metaphor of killing mockingbirds, and thus makes a good title. If you've accidentally written yourself in a title drop, good work!
Any combination of the above
Harry Potter and the Philosopher's Stone? Main character plus plot device. The Lion, the Witch and the Wardrobe? Protagonist (of sorts), antagonist, and plot device.
How do you go about titling your novels? Do you have any tips?