But then I was in yet another 'how to sell your stuff' panel (I went to all of them) where an author proudly held up her book and said, "The most important thing with a cover is that it looks professional, and that might mean you have to pay for it. I paid my cover designer a lot, but it was absolutely worth it, look at how professional my cover is!"
And in the back of the room, I was thinking, for the nth time that weekend, lady, I'm glad you're happy, but everything about that cover reeks of "self published".
It was a fairly attractive cover! The art had been done by somebody who grokked art! But not by somebody who grokked what pro-published book covers look like.
Now, I don't work in pro-publishing or graphic design (although I do some work for a very small indy publisher/art studio that sometimes does graphic design) but I work at a library and spend at least five hours a day with books passing through my hands. And then on my days off I go to cons or used book sales and look at more books. And I've reached the point where about 95% of the time, I can look at a self-pub or small-publisher book and immediately tag it as not from a pro publisher. (The other 5% of the time, it's either a self-published book by someone who really knows design, or it's from one of those small imprints of a pro house that is deliberately trying to look art-house.)
So I started thinking seriously about what it was, that je ne se quois that I could see and the people on the con panels clearly couldn't, and then testing those theories against my knowledge of design and the book covers I stare at all day every day, and here's what I came up with.
Now, I'm not saying that 'looking pro' should be your primary goal - what do I know about selfpub markets; I suspect in some of them looking pro doesn't help (niche kindle erotica, say). I also make no promises that looking pro will help you sell; it's what the marketing departments of the big publishing houses think will sell, but are they right? I can't say. Also, even within the large publishers, some genres are more flexible with these principles than others (especially on mass-market paperbacks.) But most of these principles are pretty basic to design, and are worth thinking about even if you aren't going for that particular look. And they all pretty much come down to one thing:
The only purpose of the cover is to get people to read the title of the book
That's it. That's the whole secret. Anything that makes it harder for people to read your title should not be part of the design; anything that makes it more likely for people to read your title is good.
(There are a few exceptions to this, i.e. if you're already so famous that the author name is more important than the title, or if you're part of a bestselling series or franchise and selling the brand is more important than selling the individual book, or if you're just so good that you can break the rules and get away with it, etc., but if any of those things apply, then you already have a pro marketing department working for you, and are not reading this post. And most of these things still apply anyway, they just apply to the author or franchise name or whatever instead of the title.)
Let's design an example. I'm assuming for the purpose of this post that you know the basics - how to open a file in a graphics program and put text on it, basically - but not really much else. I'll use my current silly fic WIP for the example just because it's got a title that's shaped well for playing with design and a theme that makes it easy to find public-domain art to work with. The WIP itself is ridiculous AU slash but it's an American politics AU so my marketing department has decided, for the purposes of this post, to market it as a novel about the vicissitudes of American politics. Got that?
OK, here are our two proposed covers. Which one looks more professional?

If you said "A", you have the same eye for covers that I do.
A is literally just black Times New Roman on an off-white background. It did not take an expensive art consultant, it took an open-source graphics editor and maybe two minutes to do, including looking up the correct proportions for a standard hardcover.
I expect one of the reasons so many people end up with really amateur-looking covers is that the art is what has taken all the time and effort and money, and it requires the magic that most people don't understand. Even if it's just paging though public-domain photos until you find one that feels perfect for your story (like I did on the second cover) it feels like that's most of the effort, so they let the art control the design, when it should be the text in control. Or, if they're going for minimalist art, they then decide they need to focus on the design to make it distinctive, and they let fancy design tricks detract from the title itself.
Now, I'm not saying you should always go for plain black text on white if you want to look professional, but I'm saying that you should always remember that the title being read is the most important purpose of the cover. If at any point you're making a choice about the cover, fall back on that: does it make it more likely that people will read the title? If yes, do it. If no, don't do it. And if your choice is cover A or B, then yes. Choose boring A.
I'm going to break it down a little more, but really, it all comes back to that idea.
The first set of principles are about the text itself.
1. MAKE IT BIG.
( Make It Big )
2. Center the title horizontally.
( Center the title )
3. Colors for clarity
( Color )
4. Your Font Should Be Invisible
( Your Font Should Be Invisible )
5. Manually Adjust Spacing and Kerning
( The Life-Changing Magic of Kerning )
Bonus Level: The other cover text
( Bonus level! )At this point, the sample covers are already both to where if I saw them, I would go, "This is either a small publisher who is almost there, or a designer at a big publishing house who was having a bad day."
Which is still miles beyond where they were when they started, and considering I spent zero money and did it on my spare time at work the day before holiday break, is pretty good.
Here's an animation of all the versions because animations are fun.

The next step in levelling-up is "how do I pick my cover art so it looks like a professional cover", but I think that's going to be a separate post, possibly posted on a day when people are actually online. Stay tuned! (Spoiler: the key is "if it distracts people from reading your title, change it.")