As the acknowledged king of indie book promotions, BookBub is the go-to site for book marketing. Unfortunately, they get so many offerings they can afford to be very picky about which ones they choose to promote. BookBub admits that they only promote about a quarter of the books whose authors apply for a spot on their newsletter. If you're one of the millions of authors who've tried and failed, you know the refusal letter is very polite, but vague. They simply encourage you to try again. The list of requirements on their site is a great place to begin, but even following every item on that list to the letter is no guarantee. Those who have been successful say it's more than just that list. The criteria changes sometimes, but there are ways to decipher the clues to the end of the tunnel. 
Cover is King
Like most other book promotions sites, your book cover design is one of the most important details you can pay attention to. Of course it should be professional, easy to read in thumbnail size, with no objectionable pictures. This is basic book cover 101. Delve further and you'll begin to see some patterns emerge. Study the BookBub mailer in detail every day. Have there been a lot of covers including the color blue lately? What about covers with dogs somewhere in view? The people who choose these covers are human, and just as apt to choose what looks good to them as the rest of us. BookBub covers tend to change like a tide, with many of them having at least one thing in common. Your job is to figure out what that thing is, and change your cover to match the trend.
Your Website
It's certainly possible to sell a lot of books without ever having a website, but not if you want to advertise on BookBub. It can't be a one-pager on Blogger, either. If you know WordPress, create your site yourself. Otherwise, get someone else to do it who knows what you're doing. Show the world a professional site with keywords in place, easy-to-grasp information, and an attractive set of pages.
Be Flexible
The more flexible you are about your book listing, the most likely it is that BookBub will agree to promote it. Sure, putting the book up on the biggest online Fridays of the year would be great, but you're better off giving them a range of dates, then planning your other promotions around the one they pick. The same thing goes for book categories. Take a good look at your book and decide how many categories it could possibly fit into. Romance, historical, western, women's stories, whatever remotely fits your title is another way to wedge your book onto the list.
Your Price
BookBub rarely lists any book with a price over $2.99, and the vast majority of their book promotions are for books $1.99, $0.99, and free. Getting money is always great, but if you're looking for a BookBub ad, you've probably got a catalog of other works you'd like to sell, too. Seriously consider going with the free option. You'll get your one book into thousands more hands, and a good percentage of them are likely to buy your other works, especially if you put easy-to-find links in the back of your book.
Keep the Faith and may the Force be with You!
