here my .02 :
try "dodge and burn" techniques. basically you want to dodge (lighten) the rider's shadow areas.
and use both to creat contrast in the dirt/overall picture.
Wram/dark colors recede, light colors advance. So you can try making the areas past the rider an even tonality, of warmer, and lighter towards the front.
Now here is the deal:
to create a layer just for dodging and burning:
create new layer.
set fill to 50% gray.
(at this point you wont see your image anymore)
double click this layer and set it's (masking i think) to Overlay.
you will now see your image again.
while in this new layer (active layer) use your dodge burn tools. set it to about 7 to 9% on the tool and use a large brush for the background and a small brush for the areas of the rider.
I am not expert by any means, but this method was taught to me, and it works well in most cases. any mistkes in the above - i dont have PS here at work to verify item names, all from memory.