For the past four years I have taken the month of December to plan both my sermons as well as the events of the upcoming year. As any Pastor knows there will be natural spikes and dips in attendance during each season. Recognizing this fact and working with it rather than against it has proved most helpful to me. For example, if I know that there will be a natural interest in a particular series I will plan it for the times of the year when attendance is already high. That way I can capitalize on our growth.
Keep in mind Churches grow 30-50 people at a time. You can have new people coming every week, even joining or being baptized and not experience any measurable numerical growth. In order to see substantial growth you have to PLAN well.
At the same time I am very strategic about when I preach more difficult texts. Typically I do so at times when I know I have mostly more core people in attendance.
Here is a blank copy of a 2011 preaching plan in excel. Feel free to modify it to fit your needs. Spend time in prayer, consider the natural rhythms of your congregation and plan well.
Here is a link to a previous post about Planning your Preaching Calendar