Whoever decided to put the flash of a camera facing directly forward was not a portrait photographer. The following portrait photography tips are meant to put some perspective on lighting.
The Harshness of Direct Flash
The reason it is not a good idea to have a flash pointed directly at the subject in portrait photography is that the intense burst of light tends to wash out any curvature or shadow in the face and makes the subject appear to be flatter than they are. This gives an unnatural look.
If you are using a boot shaped flash, the mount should allow you to adjust the direction of the flash. Angle the flash upwards and bounce the light off of the ceiling. This will diffuse the light and result in a more natural and appealing looking portrait.
If your flash direction is not adjustable, you may have to use a physical object to diffuse the light in the same way that a lampshade diffuses the light coming from a lamp. There are many mountable diffusers on the market. If you are at a get together and do not have a diffuser, a semi-transparent plastic cup can work in a pinch.
If you have time to prepare the area that you will be shooting the portrait in, you may want to bring along a standing light source such as a soft box. A soft box is simply a white box, often framed with wood with either thin muslin or rice paper stretched over it. There are quick collapsible ones on the market but they are pretty easy to make.
This box acts as a diffuser for a permanent or flash/strobe light that can be moved to different positions in order to get rid of or accent shadows while still resulting in a more natural look.
Unless you are shooting in a studio where you just have the subject sit and smile after the lighting has been preset, you will want to be able to move your light sources around. Many boot flashes offer remote capability so they can be removed from the camera and held to the side. This capability is great for shooting semi-candid portraits.
When shooting outdoors, your light source may be the sun. Though they often require assistance, light reflectors work great for manipulating the lighting here and softening the harsh shadows that direct sunlight often creates. Again, there are many of these on the market; I actually use a collapsible light reflector that is intended for car windshields.
I hope these portrait photography tips have helped explain the effects of direct lighting and how it can be easily manipulated for more natural looking results.