First and foremost, one should never look directly at the camera. Stare into the distance like you are watching the most interesting thing you've ever seen in your life (even if it's just your old mamabean). This requires some acting skill. Posing isn't as easy at it seems.
Hold your head up high and open your eyes wide. This will maximize your handsomeness.
Make sure your ears are always pointed forward. Twitching ears don't make for a serious portrait.
Smile, but not too broadly. Your smile should be enigmatic and mysterious. Make them wonder what you may be thinking.
And last but certainly not least, ALWAYS LEAVE THEM WANTING MORE!