Heroes Die Young was released in 2008.
Friends in Deed and Seeker were both released in 2010.
Dead or Alive, The Cure, and my first Triple-Shot (Dead Man's Forge) were all released in 2011.
Death Brings Victory and my second Triple-Shot (Temptation) were released in 2012.
My third Triple-Shot (Downtrodden) was released in 2013.
Total sales across all of my titles (again, only through 2013) is 645 since Heroes Die Young was released. A rather piddly sum, I know, but that's life. All of that in mind, I've had the following sales per title per year (for convenience, I've also included total sales per year.
So, I hesitate to start making conclusions based on these stats, since it's been proven over and over that sometimes blind luck can cause sales. And looking through the numbers, there doesn't seem to be any rhyme or reason to sales leading to more sales (of later books in the series, etc.). However, even with all of that in mind, at least one thing seems to stick out.
Generating new material for publication can generally help sales. I had three releases in 2011, and saw a marked increase. I followed that up with two more releases in 2012. With those five new titles, my sales in 2012 were five times higher than in 2010. Granted, in 2013, I had no major releases (other than the third Triple-Shot), so my sales dropped off last year. So, not only is it necessary to continue to produce new material, but it's also important to continue to do so on a regular pace. This is something I struggle with, myself, and need to work on. Hopefully this can help motivate me...