This requires quite a bit of creativity and will stretch you unless you do this for a living.
IT’S NOT ABOUT YOU!
This is where you have to think like a customer. What’s in it for them if they buy your book? What will entice (interest) them?
Don’t give away the ending! Yes a few people want to know before they read, but the majority don’t want to know. Play the numbers and to heck with the people who peek at the end.
Use action verbs whenever possible. This isn’t the place to be bland. Do your research. Look at the back cover blurbs in your library and on the successful books on Amazon’s lists. Try to figure out how you can use their ideas and apply it to your book.
There’s no formula that I’m aware of. If you find one that works, please share it with the rest of us. The nice thing is, if it doesn’t work, you can change it on Amazon and Smashwords (but probably not as easily on your paper version back cover, if any).
Pass your ideas past someone else for comments. Maybe more than one person. Other eyes and brains can sometimes take what you’ve written and make improvements. THE FINAL DECISION IS YOURS!
Now, make a few changes or additions to include some of your key words. The ones people may be searching for. This will help to improve your position in the search results. Things like “A great military science fiction action adventure.” There are 4 key phrases in that one line: military, science fiction, action, adventure. I’d use that for any of John Bowers’ books in his 3 series.