When was the last time you walked into a book store and decided to just walk around and browse through the books? If you are a book buff it probably wasn't that long ago. Even when you are browsing books there will be those that tend to catch your eye, but for what reason?
Obviously there is something about a particular book cover that catches your eye and makes you want to take a closer look. It could be that the colors of the book cover stand out from the others on the shelf. 
Or possibly it is the image that was used in the design? It could be something as simple as the title itself, did the words make you stop and look twice?
All of the above points are important considerations when it comes to designing a great book cover, either for a printed book or for an ecover. The next time a book or magazine makes you stop and look twice, make a note of what it was that caught your eye. You can then use this same element in your own designs.
This doesn't just apply to books, walk into a music store or even an art store and look for designs that pop out at you. Take a note book with you and make notes of when this happens and the reasons why. If you have your Smartphone with you take a photo of the image, this way you can study it more closely when you get home.
Remember to take a look at the bestseller lists and see if you can discover a common theme between all the books. Do they all have a similar design, do they all use images or are they are very simple and just use text?
While the content of the book is important it is the cover that is going to do the enticing. If you are not intrigued in any way you aren't going to bother to pick up the book and leaf through it.
If you take the time and research book covers and study their designs you will have more success with your own ecovers. If you use an online software to create your covers, as I do, then you will have no problem with creating multiple versions for your book. You can then display them side by side and see which ones attract your attention the most.
Don't forget to ask your friends and family for their opinion too. Just get them to take a quick look and see which ones hold more appeal. This will help you discover the best ecover for your book.