This may sound odd, but if you are going to be standing and playing in church, stand up when you practice as well. More than a few times I've worked on a song or technique whilst relaxing in my favorite chair only to find things aren't quite landing in the same place when standing up

Might not hurt to get used to your amp being sidewashed or placed behind you either...

oh, and don't forget to breathe.

Best bit of advice I can give....don't stop. No matter what keep playing. If you make a minor mistake, chances are if you keep going the audience wont notice. I know you'll fret over it but they probably wont, unless you stop or lose track of time.

And obviously just try and enjoy yourself, its a huge thrill. First time I played live I almost backed out due to nerves, but once I was up there and forgot about the audience it was just me and my guitar making some sweet tones, then it was over and I had to go. All I wanted to do was get back up on stage.

I figure that out also , strumming is not the same standing !