If you're an author interested in self-publishing, you have to think the way a publisher thinks. The number one focus for any publisher is profit. Before you even start the writing, you have to ask yourself if the idea is marketable. Once you feel you've found something to write about that could sell, you can start writing.
Make whatever you're writing the highest quality work you are able to generate. Edit it as many times as needed to condense your writing, cut back on punctuation, and make it easy for anyone using mobile devices to read your work. If you don't feel that you have what it takes to edit your own work, pay someone to do it. It could be the most valuable investment you make since a great number of books are shunned by the public for bad grammar, spelling, or poor writing in general.
Get your work out as quickly as possible, especially if what you're publishing is a sequel or not your first book. In either case, your readers will be waiting to hear from you. The longer you keep them waiting, the more likely that they'll move on to another writer and forget you. Remember, the general public has an unlimited supply of reading material to choose from. Everything you do to make yours stand out will have positive consequences in the long run.
Take advantage of everything available on the Internet for publishing your work for free. We're all getting familiar with Amazon's platform, which is looking to be the leader in terms of payouts to authors of all levels, but don't discount some of the smaller self-publishing venues like Lulu and CreateSpace, both of whom offer competitive author benefits.
Finally, don't rule out Indie publishers. If you want to go the traditional route, but don't feel that your story has what it takes to make it in the mainstream arena, an Indie publishing house could be your solution. These smaller houses are focused on building niche markets for their readers and authors alike. They are much more likely to accept work that other publishers might not see as commercial enough to warrant their attention. The really great thing about using a smaller publisher like this though is that authors are sending a clear message that there are alternatives, viable and lucrative, that they're willing to take over the big-box publishing establishments. That translates into a better playing field for everyone.