Not publishing on Amazon. Seriously, sites like Smashwords or Authonomy can be great resources, but not posting on Amazon is like shooting yourself in the foot. Sales require exposure and exposure requires you taking advantage of the tools at your disposal. Seriously--the biggest online retailer of ebooks will feature your work. There is no reason in the world why you wouldn't take advantage of this. Maybe you're trying to protest Amazon's stranglehold on the industry, which is a noble gesture--but if a big name publisher came along and offered you a six-figure advance on your next book, which would be sold all over the world--including on Amazon, you'd jump on it. Anyone would. So please, put your book on the Kindle Store. It takes half an hour.
Underselling. I've had people send me emails advertising an 'archetypal' fantasty novel or saying 'It's a YA paranormal romance . . . like there aren't too many of those, right?" What are these people doing? Trying to tempt me into reading their book by lowering my expectations right off the bat? In the words of one of my favorite authors, "only a fool humbles himself when the world is so full of men eager to do that job for him" (props to anyone who can ID this quote!). You might think you're just being cool about your aspirations--"Yeah, I know I wrote a book about vampires screwing each other. Like that hasn't been done before. Whatever." But it comes off like you're embarrassed of your own design. If the author isn't a fan of their own work, why should anyone else read it?
Third person omnipotent voice. This is more of a pet peeve, but I've never seen any author do it well. I'm not talking about a third person viewpoint with a different narrator each chapter, but I hate it when authors jump from POV to POV without indicating the person has changed. If we see a minor character's innermost thoughts establish a little bit of exposition and we don't see that person's thoughts when it's really important we see them, we're going to wonder why. Third person omni takes us out of a character's viewpoint and forces us to see what the author sees. It jolts us out of the character's thoughts, distorting our internal rhythm. If you want to use multiple POVs, put them in different chapters or use line breaks to indicate POV switching. Switching without warning is annoying.
Cover design. I've seen some horrible covers in my time. It's never good when an author whips out Microsoft Paint to try and design something marketable, but I've seen covers that are just as bad from professional artists. Here's what you should do: email the nearest college that offers a graphic design program and advertise that you're looking for an artist to design a book cover. Request that all artists show a portfolio of at least three pieces they've designed themselves. In the meantime, go to your bookstore and look at the section where your book would be shelved. Look at the covers. Pick the artist whose portfolio closest fits that style and send them links to the covers of other books in that genre. Anyone who charges over two hundred dollars is charging you too much.
Use of Third Person. I can't tell you how many times I've gotten emails saying " 'XX' is an accomplished author who's won the X prize for Short Fiction at the X-town literary festival". If you're a representative of the author, why not say "As the representative of 'XX', I, 'YY' think this book would be a good fit for your blog'? My worst fear that this is really just the author writing in third person to make me think they have 'people'. I don't care if you have people. I care if you sound like you have some strange mental disorder. Also, I can tell if you wrote the bio on your Amazon page yourself--because if you wrote it in the third person, you're going to sound like a lunatic. Look at this excerpt from the Amazon page of Scott Colby, whose book Shotgun I genuinely enjoyed.
'Scott Colby has been writing for a looooooooooong time. It started back in fourth grade, when Scott got really bored in class and decided to entertain himself by writing ten page epics about he and his friends. He soon got caught and was forced to read his stories to the whole class. Undaunted by this heinous punishment, Scott kept on writing and soon was cranking out dozens of pages of prose every month.'
You can tell the author wrote this himself, right? Why on earth do we need to think that someone else wrote it? You're not going to make anyone think you're a professional writer--it says a few paragraphs down that you work in IT!
And if you've got someone sending emails for you, introduce me, because I could use that.
Doesn't your book deserve the very best? Just take my advice: it probably does.