I've found multiple tutorials on how to take a shape and turn it into a silhouette, but full black sometimes lacks expression. I was wondering what was the best way of preserving highlights with Photoshop to create an image like the one below.
Levels are your friend here. Lets take this image of a very happy lady:
Next stage we convert the image to Grayscale (Image -> Mode -> Grayscale).
Finally we adjust the levels (Image -> Adjustments -> Levels...) bringing the lighter areas to darker shades. Here I had to apply it multiple times as there are a lot of lighter shades around her facial areas I wanted to remove whilst allowing some of the areas on her legs to stay. I first adjust the levels to the degree I want to maintain highlights at, then grab sections with the marquee tool and adjust their levels further until they are fully blacked out.
Very quick but effective, I'm sure you could create some great effects if you spent a little more time on it.