I've never claimed to be an authority on this whole self-publishing/author thing, but let's talk about the ways I've done things in the past that didn't work. I can show you my mistakes for learning purposes. I'm almost always an example of "don't do this." A cautionary tale!
This isn't my first book blurb.
Over time, it never gets any easier to write a book blurb that makes sense, pulls the reader in, and doesn't give away twists within the book. That last part is the hardest for me. How do I snatch the reader with a great hook without giving away the surprising plot points?
The blurb is the few sentences on the back cover that is supposed to make a potential reader open the pages to read more. They have to think, "OHMIGOSH, whatever will happen next?!" or something along those lines. It has to be compelling!
How to NOT Write a Book Blurb
Bore the Reader
Don't write a book blurb so short and lifeless that it bores the reader to death. If the reader can't make it through the first sentence, they definitely won't open that first page. It has to pull them in like a fishhook through the lip. Without having met any of your characters, they have to be invested enough or curious enough to read further. How much pressure is that?
Condense the Book into the Blurb
The blurb shouldn't be a condensed synopsis of the book itself. Readers don't want to read a book report. Remember book reports in middle school? Imagine that on the back cover. Nobody would ever read a book again, even a classic, based on a book report.
Throw Together a Blurb on the Fly
The blurb should be carefully crafted and as edited as the pages inside. My blurbs fall into this category. I find them extremely difficult to form perfectly. I agonize over writing them so when the time comes, I shove a few sentences together and call it complete. (Blurb on the Fly sounds like a crazy mixed drink. It would definitely have bourbon in it.)
Never Research Other Blurbs
If you're writing for a science fiction story, it shouldn't sound like a romance blurb. There are certain blurb styles that work within a genre, and you should research popular authors in yours. Find out what they use, and why it works.
At one time or another, I've done these things. As a self-published author, for better or worse, you are the only person contributing to the success of your book. After completing your own writing, editing and polishing, the blurb can't be tacked on the end as an afterthought. Aside from the cover, it's the most important part of the book. Amazon has 2.4 million ebooks in its Kindle store. That book blurb better be compelling enough to compete.