Open your image in Photoshop and make a copy of the background by typing ctrl+j. Photoshop will name the new layer "Layer 1" which you can change if you wish to something like "Contrast" by double-clicking the layer name and typing the new name. For this example I'm using a photo of a white dahlia flower that I took.
Now change the blending mode to Soft Light and you have an image with greatly enhanced contrast.
Sometimes the shadows may end up being too dark, so you can bring them back a bit by changing the opacity of the layer. In my example I changed it to 75% opacity.
That's it! You are done!