Stick with what you know, or practice before you do a really important shoot. Different color skin means different exposure, same goes for hair. That's one reason I use separate light meters. They won't be fooled by these different situations. I'm not going to go into posing techniques and theory here, just remind you that you should try out a variety of standard facial expressions.
Some people have one smile they present to the photographer every single time.
I'm the type of person who has a thousand expressions, and getting two photos with the same expression is almost impossible. Others have a few expressions that they can summon at will. Much easier, and you'll waste less film. Be aware that a two dimensional photo and what you saw when you took the photo are two different things. That's why the finished result is often quite different from what you imagined.