At the T4G conference I had the privilege of being able to attend John Piper's table talk after one of the evening sessions.
I was able to ask Piper a question about preaching, and he gave me a very helpful answer.
My question was about what I should do when I've written my sermon, its very late Sat night 1-4 am., and I don't think the sermon is very good. Should I keep working on it, or should I go to bed.
Piper said that the important thing is to make sure that you have the truth of the text, and then some general application, and then go to sleep, and ask God for gifts.
By ask God for gifts, he meant asking God to give you the words to speak when you preach, words that will apply to the congregation.
I found this very helpful advice.
The way I apply this now is that I feel happy to go to bed, as long as I have exegeted the text properly, and have a general application.
This is a real blessing for me, because as some of you know, I do not get much sleep!