Ever had a situation where you were trying to manual focus on something that was very difficult to see in your view finder? Maybe it is a subject very far away, or maybe you are using a very shallow depth of field and you can't quite tell if the exact plane you want is tack sharp.
If you have a relatively modern DSLR, you switch your camera into "live view" mode. In live view you can now see your composition on the back of the camera. Now, using the zoom feature on your camera (much the same way you would zoom on an image after you shoot to check focus), you can zoom in super close on the area you wish to have tack sharp. Adjust your manual focus ring until you've got what you need, then turn off the "live view" mode.
Shoot your photo and you'll have perfect focus, even on subject which you can barely see in your view finder. Works like a champ.