San Serif fonts (i.e. Arial, Tahoma, Calibri, Swis721, Verdana, etc.) are easier to read than Serif fonts (i.e. Times New Roman, Courier, Garamond, Palatino Linotype, etc.). Always use good contrast (in other words blue font on green background is hard to read). Better to be white on dark than dark on white (for screen), the opposite is somewhat true for print. Of course there are exceptions for both. There is a mathmatical rule for font size depending on size of screen and farthest distance viewer is from the screen, but I unfortunately I can't remember it off the top of my head. Free space (aka white space in print) is your friend. Too much text on a screen makes it easy to get lost.
If you are putting text over a picture and the text color blends too much with the picture, put a semi-transparent darking matte behind the text to bring the text out more. Adding a drop shadow and small outline can help, but be careful of over doing it. Also if it is a busy picture (say a shot of tree with leaves) that competes with the text, blur the picture a little and it will help the text stand out. If you don't want to blur the entire picture, again do a matte around the text portion.
Here's a sample of slides used in a recent sermon:
http://s449.photobucket.com/albums/q...onSequence.mp4
And a static announcement slide:
