This is especially true of a long pastorate. We all know that long pastorates are good for churches, but they can expose a serious weakness in your preaching. Back in the old circuit riding days, a preacher who had a dozen messages on justification through sanctification and had a week's worth of revival messages down pat could get through an entire career. He'd just hone the messages over and over as he went from charge to charge. But when people really get to know you, they get used to the way you preach; in fact, people remember my sermons better than I do (a compliment to me, I think), so I have to be careful not to repeat myself too often.
As busy as preachers are, it's tough to find time for serious, challenging Bible study. Though we don't like to admit it, most of us do the lion's share of our Bible study as part of sermon preparation or when we're preparing to lead studies. So setting yourself a challenge now and then is important.
Many of my colleagues use the officially prepared lectionary to plan their sermons. That was just beginning to be employed among Protestants back when I was in seminary. I was trained in an older* method. When I sit down to plan several weeks of worship, I have to ask myself what needs to be said, what the liturgical season is about, how do I balance Old and New Testaments, what Scripture will be preached from and what Scripture will be used liturgically, what hymns will be sung, etc. In effect, I create a mini-lectionary for my own use several times a year. It's a lot of work, but it stimulates the mind and spirit. Too many preachers take the prepared readings and punt on the study it takes to understand why they're put together the way they are and just do the same ol', same ol' with a new text and title.
* Older among Methodists and other evangelical, Free Church types. Lectionaries go back to at least the days of the late Roman Empire. I know that, so you liturgigeeks out there don't need to be bustin' my chops about it.