To write your novel, you have to know the story. To sell the novel, you must condense that knowledge into a pitch. Novelists are generally not blessed by the Brevity Fairy, which is why book proposals and synopses are so universally despised.
There are techniques that help a writer organize novel info, summarize it and build a decent pitch. Practice using catch phrases, buzz words and other premise-makers to hone your pitching skills. Internet marketing author Joe Bingham has a good article about catch phrases here, and Bombshell author Carol Stephenson has one on romance buzz words here.
These are some writer exercises and examples I've used in classes and workshops to teach the art of less is more:
1. Write a classified ad selling your novel.
Dark hi-speed killride: Nightmares, get real in this standalone urban fantasy novel, 150,000 words, drifter female mechanic/mystic protag, present-day Chicago/otherworld settings, apocalyptic stakes. Seatbelts and safeties have been removed.
2. Make up a motto based on your novel theme.
Life isn't Death's bitch anymore
3. Describe your novel in 15 words or less.
Aggressive aquatic pilot/gunner guarding troubled peace summit must avert diplomatic assassination and interstellar war.
4. Write a personal ad for your hero or heroine.
Not Desperately Seeking Anyone: 35 y.o. swm, 6'5, 230 lbs., Fire Marshall of New Orleans, serious, GQ dresser, sometimes grim, hard worker, temporarily living at home with wealthy parents, tired of beautiful, empty-headed socialites. Chasing an arsonist/mass murderer who has vowed to kill everyone I love, so discretion a must. No cops.
5. Come up with a single hook line that describes the opening of your novel.
All I was trying to do when they caught me was bury my mother in an unmarked grave.