In 2015, I intend to release every month, work on only one series at a time, focus on the sub-genres that are selling well for me, set a pre-determined number of books in the series (3-6), only adding to the series if it's selling really well.
I made some costly mistakes with my series this year. I worked on two series simultaneously and released only bi-monthly. This meant readers had to wait up to four months for a release in a new series. I found that was too long for my readers. I intend to work on one series at a time from now on. In my case, one series was selling well, the other not so well. I shouldn't have committed to writing six books in each series until I figured out which was going to work.
If I had it to do again, I'd commit to three books, then determine if I wanted to add more based on sales. I love to write long series, because it allows me to create a world surrounding a large group of families and friends, but that strategy only works if the series is selling really well. From now on, I'll move on if numbers aren't where I'd like them to be after three books.