When a business of any kind is planning its marketing the most important thing they must ask themselves is "Who is my target customer?" In order to market correctly you have to know who it is you are selling to. For a writer this can be tricky however because unless you are writing only one specific type of book your audience may change with every new publication. There is certainly a different type of reader for sic-fi than for erotica. Romance readers tend to be very loyal to their genre and wouldn't necessarily pick up a random thriller.
As an author it is vital to know who would buy each style of book you may create so that you can tailor your marketing plan to that audience. By presenting images and teasers from your books you can draw the reader into the story but you wouldn't want to post something frightening to a group that buys children's picture books. I have a number of books and have files for each genre with subfiles for each book that fits into that category. I am still learning the how of making teasers and promo pieces but when I figure it out I will also keep them all separated.
Another thing to keep in mind is when you do a promotional event the audience is one of the biggest factors to identify. A few months ago I held a book launch at my work, The Venetian Hotel & Casino on the Las Vegas Strip. The book is about domestic violence and I partnered with a local shelter I support very strongly. The Shade Tree celebrated its twenty sixth year along with Domestic Violence Awareness Month and in conjunction with Sands Cares, the philanthropy side of Venetian we held a raffle, shared a reading and even enjoyed a delicious dessert while launching my book.
I had to keep many things in mind while planning the event. I had to know who my potential readers were in order to spread the word about the book. I had to remember things important to Shade Tree so as not to offend them or those they worked with. When gathering raffle items and working with different promoters I also needed to focus on stores and restaurants that were within the Venetian so as not to create a conflict of interest.
It can be difficult to know exactly who your audiences and how to reach them but if you take a moment and ask around you can find out what your friends read and build on that. Go to book stores and watch as people browse the shelves. If your are outgoing enough strike up a conversation with other book lovers or even ask the cashiers about who buys what. You can go and observe readers at your local library as well. Once you have a basic idea you can begin to look at the covers of the books. What makes them appealing? What books seem to make people stop often and which ones are passed by all the time? Identify a few books and look them up online to see what kind of advertisements have been created for them. Are they mass marketed to several types of people or are they specific? You can build your personal database book by book and reader by reader to help understand who to market each of your books too.