What camera / lens will you be using?
Main thing would be to shoot in a spot with consistant light ...
I'd suggest going to Shutter Priority (Tv) and set it to the minimum acceptable speed. In tight woods, with them coming towards you or away from you, a lower speed will still stop action. See if you can get away with ISO 400 and no slower than 1/500, slower may even work, but don't let the shutter speed drop below the length (mm) you are shooting at.
Shoot RAW to allow exposure adjustments after the fact.
With experience? Go full manual, shoot a white card in the ambient light, set custom white balance and maunually expose to changing conditions. It's pretty easy for the in camera metering to be fooled.
Here's a shot of DRN'er Fatherandson from DirtWeek 05. It was pretty dark in there, but I found a spot where the subject was "all in" the shade. Shot at ISO 400, f/2.8, 1/1000, 70mm. I metered on the ground and locked before he got there (metered on the ground about where his boot is).