One of the best ways to increase sales is to sell another book. With two books out, you can potentially make twice as much money. With three books, three times as much. The math is simple. Take it from me, I'm a PhD math student! :)
But this means you have to write another book. So, unfortunately, you can't spend all of your time marketing. Once again, learning to budget your time is very important. On top of all my other things to do, I like to give myself a daily writing goal of 1000 words. From time to time I will join challenges that increase that limit (i.e. Nanowrimo). With a writing goal of 1000 words per day, I should be able to get a new book out every three months or so after editing and cover design.