If the ceiling is white, and not too high, you will probably get best looking pictures if you angle the flash at one step less than straight up. This will diffuse the light and fill in a lot of otherwise annoying shadows. If the flash has a catch light card, pulling it out will give enough reflection to get nice reflected highights in eyes.
If you are outside, or in a place with high ceilings, use direct flash and the pull-out diffuser (if it has one). Have spare batteries for the camera and flash, and enough memory cards. Good Luck!