This cannot be stressed enough: if you are unable to summarize or tell something about your book (or product or service) in less than thirty seconds, then you need to sit down and work on this.
There will be many times when someone will ask you, "Hey, what's your book about?" And if you can't answer them in about 20-30 seconds, you've lost an opportunity.
Spend a little time on this. To summarize a 400-500 page novel into a handful sentences is difficult. Trust me. Been there, got the T-shirt. Besides, ever read the jacket cover of a book? Of course you have. There are people who all they do is write them.
The best way I've seen--and I stress that this is just me--is to go completely away from the computer and either go for a walk or a drive. I may just sit quietly somewhere, thinking about the overall story. Then, write down everything you can. It doesn't matter how much, because you're going to whittle it down later.
Because the more you write down, the more you have to choose from.
Then, once you've got it nailed down, practice and practice and practice.