This is the shot that I took 2nd place in the Teva Mountain Games with. My lovely assistant/girlfriend (Kelly) helped carry my 18.6 pound alienbee battery pack and flash head halfway up Vail mountain. I highly recommend getting an assistant of your own (especially if they don't charge....)
Forest lighting is usually very spotty. This can create very difficult situations. My best tip for forest shooting with natural light is to either shoot your subject in a long sunny stretch, or capture them in a long shady stretch with a dark background. But that's just not good enough for today's style of photography! We need flash, and more power is always better (just ask Tim the Toolman Taylor from Home Improvement) This is why I choose to use the Alienbees B1600 flash head and Vagabond battery pack. The whole setup costs about $659.
I try to make the subject pop from the splotchy background by using my flash on the athlete. This way, I can keep the light and shadow spots on the trail, and still have a bright athlete. Strobes are balanced to 5800K which is equivalent to daylight. The next step is to line the flash up at an angle similar to the sun's direction. This will create the most natural look.
When I started taking shots with this flash setup, I was planning on capturing one rider in the path of the flash. During a race you can't really plan too well, because athletes will be at varying speeds. It's always good to start with a plan, but you can never expect things to go the way you hope. Luckily with this shot, this was not the case. The riders were close, and I lucked out with my timing. The one in the foreground is lit by my flash, and the one in the back is natural sun. Perfect timing! The stars aligned for me on this shot!
A light meter is a great tool to create an even light with ambient and flash. It also saves a lot of time. I did not have one for this picture, but was able to balance the lighting with enough test shots. I can't recommend the light meter enough.
To close, I have one final tip (if I have not annoyed you with too many already...) As a professional photographer, your light is what will give you the edge. Flash will bring your photography to the next level, and using it in unconventional ways will make you stand out from the pack!