The original image is already perfect - why this demo? Well I raised the idea of an improvement for a perfect image, so why not give it a try anyway to share some technique. Still you should note that my skills are very basic and very likely there a easier ways to achieve the same
Some steps for this modification.
1. Copied the image to new layer
2. Pushed the new layer to enhance the dark parts a lot.
3. Selected the reflected parts of the flower in the pushed layer to see the borders.
4. Copied the selection from the original layer into a third one.
5. Adjusted (pushed) the gradation curve of the third layer.
6. Layer 2 deleted.
7. Opacity of the third layer can still be just for fine adjustments.
8. Saved merged layer into JPG
Hope it is clear enough!
The original is here: http://www.pbase.com/stfchallenge/image/64584237