In a past life, I worked as an assistant marketing manager for a fashion designer (his perfume line), where I lived and breathed the 4 P’s of Marketing: Product, Place, Price and Promotion. (There’s a fifth P as well—Planning—which I address as well).
Here is a brief recap of what I know and how an author can apply it to their own promotional plans in the digital age.

Product/Service
Who is your target audience and how does your brand satisfy their needs? Your books are part of your brand. But don’t forget, YOU, the author, are also your brand. Strange, right? But put your fan-girl hat on and think about authors you love. Do you know what they look like? (Probably yes.) Have you visited their Facebook page or Website, or have you signed up for their newsletter? (I know I have.)
Think about yourself being the consumer of their books. Yes, you love their writing. But you also are buying into how they’ve branded themselves.
Check out the marvelous Susan Elizabeth Phillips Website. In the first paragraph of her Dear Reader letter, she restates what she knows about you, the reader. What needs you have that she is able to satisfy—the poignant, often hilarious stories she writes. She’s not talking about one book in this paragraph, but the general themes that readers seem to respond to.
Lesson Learned: KNOW YOUR READER. 
Place
Where are readers going to buy your books? This has a lot to do with your publisher and/or whether you are an indie author or traditionally published.
For authors, it is more about accessibility of purchasing your books than hiring a sales force to promote them.
Digital sales: Make sure your buy links are visible on your webpage. Who do you do the most sales with? (Ah, humph…Amazon…)?

How are you going to draw readers to the purchase site/link?
Facebook ads work, by the way. I just completed a week long ad linking directly to the online retailer I felt I had the most sales potential with. The key to Facebook ads is you must target your reader (yes, you need to know who they are ahead of time). For example, I indicated on the Facebook ad setup page that my audience’s interest were other authors, like Katy Evans and Sylvia Day. So their Facebook fans received my Facebook ad.
Bookstores: My books will be in stores in March so I don’t have much to say about placement. This is something your publisher handles. There is the whole crew of Indie authors (many who have been picked up by publishers) who are getting their books into Target and Walmart.
Lesson Learned: MAKE IT EASY FOR READERS TO BUY YOUR STORIES
Price
You may or may not have control over this. However, if you are an Indie author, you have some flexibility in playing around with your book price to see what best works for your readership.
I’ve heard that a typical pricing structure for when your third book in a series is released is:
Book One: .99 cents (or free)
Book Two: $1.99 (on special price)
Book Three: Full price ($2.99 – $4.99)
The trend in reading is to give the first book away “free”. I just remember a discussion my team had with a former Vice President of Marketing about free=cheapening your brand. That anything labeled free subtly suggests it’s not “worth” spending money on.
That said, it seems like more authors than not are offering their work for free in the hopes of hooking a reader through their writing. It makes sense.
So never say never.
Lesson Learned: PAY ATTENTION See if in offering your book at a lower price/free affects interest in your other books.
Promotion
When and how are you going to read your target readership? How are other successful authors with the same readership promoting their work?
It is hard to measure promotional success as there are a lot of factors feeding into why a reader buys a book the very moment they buy it. In my opinion, it all boils down to this:
High Visibility/Buzz is the key.
The more visible/discussion about your book is on multiple platforms, the better. Remember, it takes the average person three times to see something they might want to actually buying it.
I found this interesting link on purchasing decisions that is worth taking a peek at: 9-things-to-know-about-influencing-purchasing-decisions/
Reviewers/Reviews: Make a list of reviewers you think might like your work and/or have readers on their blogs who might like your work. Target those reviewers, offering them an advance copy along with a blurb and synopsis.
Facebook Ads – see my experience above
Blog posts – Try and target the major blogs (HEA, RT, etc.) Blog tours tend to be a time suck.
Join forces with other authors – This worked out really well for my second release, Tap Out. Five other sports authors and I wrote joint blogs and promotional blitz pieces for our books. (We had similar release dates, which helped.) We were approached by major reviewers to do a themed post, which was both fun and I think interesting to the reader.
Tweets/Facebook Posts/Goodreads blog post – Have a team of people who will help you get the word out. I am fortunate to have another author who actually writes the tweet posts for me (and other authors published with my publisher). Every week, we’ll tweet in support of each other’s work.
LESSON LEARNED: GET THE WORD OUT
Planning
Yes, you need a plan, especially if you have more than one book releasing/released. Most marketers will use a good ol’ fashioned calendar. Write in the release date of your book then backtrack. What kind of promotion are you planning on doing? Fill it in. What events are you attending? Fill it in.
The greatest feature in Facebook and Twitter is the SCHEDULE POST option. You can write short promotional clips ahead of time and schedule exactly when you want them released.
I sit down (per my publisher’s instructionsJ) and complete a few pages of text I want to release. Remember, you are not selling your books, but your brand. So make sure you include tidbits about yourself, what inspires you, what you find funny or interesting.
LESSON LEARNED: ADVANCE PREPARATION HELPS.
Remember, the 4 (plus 1) P’s of marketing the next time you put on your author-now-in-the-know hat!