there are several good masking techniques.But I generally use the pen-tool technique.Outline the subject , then go to path>make selection.Presto ! You could go and use more elaborate methods for subjects with hair, but I believe it's a waste of time, since details will get lost anyway.The pen tool ensures flexible usage and if you have difficult areas like hair, you can trick them out by using the smudge tool at 80-90% strength and 900 fading.Hope this helps !