Don't email your mailing list with "hey read my column tomorrow" Send the email ONLY when your column is actually available to be read. Sending a link to something people can't see is frustrating to your readers and a complete waste of time.
2. Linking your contests and promotions to the number of followers you have or want.
No one but you cares how many followers you have. Link to something your fans care about. Or just have a contest for no reason.
3. Putting too much information in your signature line.
Promote ONE book or have ONE blurb, no more. Rotate often.
4. Retweeting the nice things people say about you.
If you want people to notice, thank the original tweeter. Your fans will be able to track the conversation if they're interested.
5. Failure to build dialogue.
One of the best ways to attract followers and get "liked" is to ask questions. Then engage those readers by answering. The point of social media is interaction. If you're the only one talking, you're missing the "inter" part of the action.
6. Talking only about yourself.
One of the most effective ways to promote yourself is to talk about someone else.
7. Send ALL.
The only email that is appropriate to send to everyone in your address book is news of your death. You will not be the one to do that. Personalized promotional emails are ideal. At the very least divide your address book into Friends (NOT the Facebook idea of friends either), Family, Business colleagues, writers and other writing friends. Each category gets a DIFFERENT email announcing your book. (It's rare you will have "agents I've queried and from whom I've received a form rejection" on any of those lists.)
8. Apologize for promoting your book, or calling your promo emails spam.
Would you buy from someone who says her product isn't really worthy of your attention? Pay particular attention to this if you are female. I see this from female authors ten times more often than men. You don't have to laud yourself, and self-deprecation can be hilarious in MODERATION, but "oh I hate to bother you with this" is just plain bad promotion.
9. Starting too late.
Promotion is a long, slow process. You'll build friends/likes/followers in slow increments. You can NOT start the month before publication and expect any serious traction.
10. Long periods of silence or non-posting on Twitter/Facebook et al.
You're better off doing five minutes a day rather than 50 minutes once a week, or worse, three hours once a month. Social media is about presence, and it's better to be present daily even if briefly than weekly or worse monthly.