March 10 - Professionalism. Have a professional editor proof your book, your promotional materials, your letters and even your emails before you send them out en masse. What? You don't think this is part of promoting your book? Think again. Marketing materials and books sent out riddled with errors have the opposite effect of whatever you are trying to achieve.
I recommend against employing editors who were/are your former high school English teacher, spouse, or people with whom you share DNA. They are likely not qualified--nor are they trained--in marketing, publishing, or your specific subject matter. And even though you are saying to yourself (I heard you all the way over here) "She/he would be brutally honest with me," they won't, and can't, give you the best advice you need for your book. Hire a book professional to edit your book. Hire an editor, marketing professional, or production manager to proof your marketing materials. Read emails out loud to yourself (I mean it! It works!).