One of the best ways to gain exposure for your books is either to join Kindle Select program on Amazon and advertise your books on special free days, or to make your book permanently free. A last option is to set your price to 99c permanently or for a period of time. These strategies work especially well for a first book in a series.
In order to make readers aware of the free/99c promotion, you have to list your book with websites and advertising services that cater for this.
Ebooksforfreeinc.com is a new site and presently they are running a free introductory promotion and will list your Kindle book for free. Have a look at the site and promotion here: