Create a Dark and Surreal Poster Using Your Own Portrait" on psd.tutsplus.net and one step really catch my attention.
Among other things author writes that when he wants to add more details and drama to his portrait he draws hair and beard over the actual photo. On the picture bellow you can see what exactly do I mean:
This is black and white example but I´m pretty sure that it will work on color portrait photo too. You just need some light brown color instead of white. To have even more perfect look paint also some darker hair ans bread.
I didn´t try it yet but once I have oportunity I will show you the result.
I also think that it´s a trick which is used in "cinematic portraits". I wrote an article about them two months ago.