1. Waiting until you are finished to start marketing your book.
If you wait to start marketing until your book is finished and on store shelves then you've waited too long. The period right before your book is released is a prime time for marketing. Give your audience peeks into your process, show the cover, post the first chapter on your blog, give out advanced reader copies, and just talk about your book. Get your website and blog primed and ready for the big release BEFORE the big release. This is one of your biggest marketing tools.
2. Don't be annoying on social media and ONLY talk about your book.
When people on Twitter or Facebook constantly link me to their Amazon page it bugs me. People don't want to feel like you're selling to them all the time. Think of it like a friendship---you don't want to be thought of as that annoying friend who can only talk about one thing and everyone smiles politely and tries to get away as quickly as possible. I try hard to let people get to know me as a person so they'll want to know me as an author. Use social media to reach out and talk about what makes you interesting.
3. Putting your marketing eggs in one basket
I can't tell you the number of authors I've met who don't have a multi-faceted marketing plan. Granted, there is no magic marketing plan that will work for everyone, but at least consider doing more than a blog tour. You can do print media (and that doesn't necessarily mean just putting out a press release of your book launch. You can use your platform. For example, I did a Skittles for Soldiers campaign that sent 900 lbs. of food and hygiene materials to our soldiers in Iraq and Afghanistan because my books are about military men and women serving in those countries. During my research I interviewed soldiers there and they told me they missed eating Skittles most of all. So, not only did I feel wonderful helping our servicemen and women, I also got some great publicity for my novels.) You can do radio (I was part of a storytelling radio program) You can do TV appearances if you have a platform. You can do blog tours, book bombs, and contests. Use Goodreads and Amazon author pages to their full extent. Use your newsletter to provide behind the scene stuff or deleted scenes and give your fans what they want. But again, don't do #2. Don't be annoying. Be well-rounded both in your marketing approach and in your online presence.
Marketing is a fine line between letting people know your book is available without overkill where people get sick of hearing about your books. It can sometimes be a steep learning curve, but avoiding the big pitfalls is a step in the right direction.
What marketing mistakes have you seen authors make?