Here is the first thing you need to know: writer's block is not real. It is just laziness. Call a dog by its name and it is more likely to come.
Laziness, though not sloth, is overcome by work. That distinction is another post. But what work will overcome laziness? Not study. Laziness loves study. It goes like this, "Oh, I don't know what to say, so I'll read more." Wrong. That won't get a sermon written. A sermon gets written by writing. Just write. That is the work that needs to be done. That is the best way to overcome laziness. Vomit on the page. Clean it up later. Just write.
Along with this vomit technique, the other great secret, is this: lower your standards. Don't try to be profound or insightful or clever. Be simple. It is okay that you say nothing new. It is probably far better that you don't. It is also okay that you bore the hearers and preach in a formulaic way. The people actually need to hear the same thing over and over and over again.
If it is the right formula - "You're bad and should go to Hell. But Jesus died for you and forgives all your sins so you won't. Now He gives us His risen Body and Blood to enjoy a bit of heaven here on earth." - God be praised. The right formula, in and of itself, is a good thing. It is needed by the people - over and over and over again. Just because you’re bored doesn’t mean the angels are.
The right formula is good and salutary. Use it. Do not be ashamed of it just because you, or some professional bureaucrat in your congregation, is bored with it. I am serious about this: I am absolutely in favor of formulaic preaching. I do it, and I encourage others to do it also. Professors don’t like it. But they also don’t preach week in and week out, nor do they have the care of souls assigned to them, souls that are not in need of clever stories and great insight and hooks to keep them coming back or following along, but souls that need the saving Gospel of Jesus Christ. That is what they need. So give it to them – even if it is boring and repetitive and all been done before. Give it to them even if it isn’t “textual.” Just do it.
Here is a corollary: when you're stuck, and anyone who suffers the burden of constant preaching will get stuck, when you’re stuck: lower your standards.
So there you have it. Here is how to overcome sermon writer’s block and get done what needs to get done: lower your standards, stick to a formula, and vomit on the page. Forget the steps and processes you were taught in homiletics. That stuff has good intentions but was dreamed up by academics. It really boils down to just this: write.
And then relax. Go home early. Drink a beer. Sermon writing isn’t that hard. There is a promise in our work. In a sense, you can’t fail. The Lord will provide. He is present in the Office to the end of the age. Vomit away. God will sort it out. And even if you give them vomit, the Lord will save your hearers through the liturgy despite you.