There are several ways to effectively use a flashlight for self defense, personal protection and home defense. There are also ways of using a light that can do more harm than good, maybe even get you shot.
Tip #1 is to use the light sparingly. Leaving your light in constant-on mode makes you an easy target to shoot at.
Another critical point is this: Don't hold the flashlight directly in front of you. Knowing Tip#1, you can probably figure out why this is important.
To learn more about effective use of flashlights in self and home defense (and techniques to avoid) check out Own the Night by Scott Wagner.
In the book, Wagner explores advances in handheld tactical lights, weapon mounted lights and laser sights, and reviews the proper application and use for each type of device, as well as techniques for enhancing personal safety and effectiveness.
While Wagner's perspective is that of a seasoned law enforcement professional, the core principles he teaches are critical to self defense, personal protection and home defense applications.