First, finish the book. This is important because you need something that captures it as a whole. Call it anything you like while it's a WIP. I used to have a file on my computer called simply "The Book".
Second, unless you want it to sound like other stuff (which could actually be a sales advantage) I'd avoid very popular title structures from other books. If I was writing crime, I would avoid "The Girl Who Was X", for instance, as there are plenty of girls on trains, with dragon tattoos etc. The equivalent in SFF would probably be fantasy novels with "fantasy words" in the title: Throne, Sword, Honour, Dragon, Destiny etc - says the man who wrote a book called Up To The Throne*.
The use of colons ("Destiny: War of the Gods" etc) always seems a bit OTT and computer-gamey to me, but if that's the done thing in your area, you might want to do it - or avoid it.
Things that seem to work for me:
- a unique object or group (The Big Sleep, Smiley's People, The War of the Worlds)
- strange - but still readable - names (Count Zero, Watership Down, The Midwich Cuckoos)
- familiar phrases, sometimes twisted (The House Next Door, Tinker Tailor Soldier Spy, The Lady In The Lake)
- some kind of dangerous activity or time (A Game of Thrones, The Day of the Jackal, A Feast For Crows)
- menacing words suggestive of violence or death, although this gets cheesy fast (Blood, Assassin, Blade etc)
All of these raise questions and suggest images. Who is Count Zero? Why are these people Smiley's and what are they like? What's so special about this house next door?
It might just be me, but repeated vowel sounds can help. I'm currently trying to write a story called The Sea of Beasts, and for some reason the repeated "ee" sound in "sea" and "beasts" just seems right.
* I justify this to myself because it's a quotation. Not the best justification, but there you go.