First thing to do is duplicate the layer. Or you can actually even triplicate it, so you will always have one layer to compare with. Then hide all other layers but the background layer by clicking the eye symbol. Next select the background layer and start to work with it by doing the normal tricks you would usually do, like 'auto levels', lightness & contrast control etc. So, what you are suppose to do is improve the base picture using traditional methods.
Once you are pleased with the background layer, then switch on and select the next layer above the background. Change the layer opacity something like 50% transparent. Then start to play with layer options by starting with something like 'multiply' or 'overlay'. Keep on eye what the results look like, and pick the option that looks best. Change the opacity some more if needed.
Finally you can switch on and off the topmost layer to see what the original looked like. Once you are happy, switch off the topmost layer (original), and just 'save as' or export the picture.