In some novels, the subtitles are a significant contribution to the enjoyment of the reading. For example, think of DeLillo's Underworld:
The Triumph Of DeathThese are intriguing -- they promise interesting things, regardless of what the rest of the book contains. [For another good example, take just about any Djam Karet album. If only Mozart had had the benefit of someone like Chuck Oken.]
1. Long Tall Sally
2. Elegy for Left Hand Alone
3. The Cloud of Unknowing
4. Cocksucker Blues
5. Better Things for Better Living through Chemistry
6. Arrangement in Gray and Black
Some books cannot benefit from this treatment; what titles could possibly be given to the sections of The Sound and the Fury? Others suffer under ignominious subtitles, notably Gardner's The Sunlight Dialogues (praised here) which is laden with 27 clunkers like "The Dialogue of Wood and Stone", "A Mother's Love", and "Winged Figure Carrying Sacrificial Animal".
Finally, we come around to a willfully obscure title which has, almost by itself, kept one of the dozen or so most important books ever written out of the mainstream. [Think before you click.] So if you want to be a novelist, write the title first. That's as far as most people will ever get, so you'll at least be tied.