Most folks assume that fonts are just a fun thing that you pick from a list, to fulfill whatever fits your fancy. But the study of typography goes back to the invention of Gutenberg's printing press. The list of fonts on your computer aren't just random styles for fun. These typefaces each communicate something, whether it's formal or whimsical, goofy or serious.
Avoid Comic Sans, Papyrus, or Impact, since they're quite sloppy and lack readability on large projection screens. Instead, use a sans serif like Avenir, Trebuchet MS, Myriad Pro, or Helvetica for an informal look. To go more formal or classical, try a serif font like Trajan, Garamond, or Quarto.
Use of color can help or hurt the readability of your slides. Darker, subtle colors are better for the background of your slide, combined with colors that pop for your text. Sometimes, white text on a black background can be quite reverent and powerful. Or choose two complementary colors from a good color scheme for your text and background colors. A third complementing color can be used for a secondary text element, too.
Along the same line as color, contrast is important between your foreground and background. Use muted backgrounds to improve contrast with content.
When you're using motion backgrounds or motion graphics, use subtle movements. The motions shouldn't distract the audience.
Don't use random images you've found in image searches. Use simple, high-resolution artwork for slide backgrounds, with flat color behind the text. If you can't find a perfect appropriate match, it's better to go with just a solid color. For instance, a subtle gradient between two very close shades of dark blue can work well.
Along with simple backgrounds and minimal movement, avoid anything in general that distracts the audience from the key parts of the slide.
Visual hierarchy means using the size, brightness, color, or thickness of elements to make them stronger or weaker than others visually, illustrating importance. You can use this to your advantage, to strengthen the message of your slides. For instance, you can emphasize a scripture passage over its reference or other text on the slide by using bolder fonts or a larger size.
A good rule of thumb is to only have one thought per slide. Where possible, break up a slide with multiple points - put each point on its own slide.
A classic mistake is to make the text too small on each slide. Test your text sizes on real slides in your church, on the actual screens you'll be using. If you can't easily read the text from the back row, you'll need to increase the size of the text.
If large text causes lyrics to spill into another slide, this is fine. Don't be afraid to use more slides. Better to use more slides than ending up with cramped content with very little room to breathe. The space between lines is just as important for readability as the size of the text.
Make sure your lyrics and sermon points are very easy to read on your background. You can make the text really pop with drop shadows, strongly contrasting colors, or the text weight. Start with a light drop shadow, and add a bit more at a time until the text is stronger than the background.