The key here is to start preparing long before you publish. Seek out as many free and cheap promotional sites you can find. Prepare a bunch of tweets, a bunch of Facebook posts, a bunch of blogs. Google a heap of sites where you can advertise your new release. There can never be too much. Send out press releases, get on your local radio, talk to your local newspaper. Try to get interviews. When you are ready to release, be ready to advertise. Have a book release. Even if your budget is limited to no budget at all, there are still a gazillion free things you can do to get your book out there. And then you need to do this every day. Day after day. Take your laptop on holidays. Even if you have a traditional publisher, the more advertising you can do the better.
Get Dan O'Sullivan's Trilogy on Amazon
Book 1 - The Fallen
Book 2 - The Guardians
Book 3 - Child of a Guardian and of the Free