A recent survey shows that the most common background color is blue. Less is still more. More is a bore. Watch for clutter. Too much text, too many graphics will appear as clutter.
Timing is everything.
Practice the transitions and slide progressions for hymns and choruses on the computer you'll be using for the actual performance. PowerPoint's transition speed will depend on the speed of your computer's processor. If you set the presentation up at home on your 233 MHz machine, then take it to church to show it on a 333 Mhz computer, your transitions will be faster, and your type effects will be quicker there. As you advance announcement slides, read them to yourself a bit slower than what's comfortable. That way you'll have the advancement about right.