
1. Know the Songs
Practice the songs many times, do not think you have practiced enough. Keep practicing up until the day that you play.
As time progresses, and you get more experience you won't need to practice as much, but it's vital that you are 100% comfortable with the songs. There will be enough things the day of that you will be thrown by, so you don't need to add to it by not being prepared with the songs.

2. Anticipate the Transitions
Look at the order of songs, are there going to be breaks in between songs? Are you going to need to play behind someone speaking? Is there a key change that you will need to perform between songs?
These things are important to learn and keep in your mind. As the keys player, transitions and play behinds are your responsibility. Practice the transitions beforehand.

3. Know what setup you will be using
It's important to know exactly what type of keyboard and accessories are going to be available to you. There are some keyboards where you will be changing the instruments on the keyboard, but if you are using MainStage or any other software then you will be doing it on a computer.
Even keeping in mind what you will have to be doing to change settings is important for when you are practicing at home. You want to remove all of the variables.

4. Become comfortable with a click... or a lack there of
It's important for you to know whether your church uses a click track or without one. There is a higher level of practicing that you will need to do if they use a click track. Becoming comfortable with a click will be hard and take time but it will be worth it in the end.
Spend all of your practice time playing along with the click, this will help you understand what is to be expected.
5. Expect failure
It's completely inevitable, there will be times when you mess up or the technology messes up and there's nothing you can do about it. But the good news is, we can prepare ourselves by knowing that this will happen.
If you constantly remind yourself that something could go wrong, you'll be less surprised in the moment. And when you are less surprised, you are able to find a solution.