Portrait With Flash

As far as I concern, there are no rules in portrait photography. It is up to you how you want the shots to be. But there is a composition guideline how you wanna make the portrait shots give more impact or we say catchy. If you are taking picture for other people be sure to know who you doing a favor. Some people especially Chinese don't like their picture taken cropped on the leg or cropped on the forehead.

Using Flash

Often there will be a time when you need your flash to take picture. In a low light condition flash is very essential for portrait photos. But when using a flash for a portrait, it can be harsh form of illumination and often creating a result of shiny on the face and red eyes. Some camera now a day had a TTL function which will make the camera assessing the amount of light available and decide the amount of light to be produce from the flash to balance to exposure. Sometime this does not always help. If you are using a flashgun or speedlight, you can bounce the flash off the ceiling or wall to softening the effect of the flash. This is by putting your flash to the ceiling or wall and not directly to your subject. So this means that you are using the ceiling or wall as a diffuser to spread the light evenly. If you have a diffuser for your flash that is even better.
A Stofen diffuser usually put on the front of the flash.
This is one of the best way to avoid the harsh light created
by the flash from creating the shiny part on the face.
If you are using a build in flash or fixed type then you could try taping a piece of white paper in front of the flash tube to diffuse the effect.
The soft effect is the key for a good portrait.