This seems like an obvious one, but it is so easy to overlook. You have to practice your instrument.
I’m not just talking about the music for Sunday, though you should certainly practice that, too. Know your instrument. Know how to build/decrease intensity. Learn stylistic things from other musical genres you can bring into it. Open up your palate and get away from just reproducing recordings. You will have a lot more options if you are more than just one note.
If you’re running band, practice your cues. Make notes of the important places to go over during rehearsal. Practice any place where you are going to deviate.
Constantly throwing shit together at the last minute is likely to going to get you shit results and kill any respect anyone has for you as a leader.