How to get the best book cover design possible to skyrocket your sales

I want to publish my own book, most of it is done already (editing etc.). It supposed to be an e-Book first, but I want to publish a hardcover version as well, if it seems profitable. I’m kind of stuck at the cover design now.

Who of you has published their own book or had to get a book cover design done by a professional designer?

A few questions to you guys or anyone else who could have an idea about it:

1.What would you value about the service of the designer? What should I look for?

2.What are some issues you faced, worries you had? Were they reasonable, or were you able to trust the designer that you hired? Did he do a good job and how?

3. Was there anything that made you regret paying for his service? Anything he could’ve done better?

Thanks for your help, would love to hear about your experiences to know how I can find the best designer possible for my work. I’m really passionate about it and I want it to be great.