I caught a tip from a photo mag a while back (I think it was from Lee Frost) - for shooting in forests (including autumn colour), overcast conditions are actually sometimes better, reducing contrast to levels manageable by transparency emulsions. His advice was load up with Velvia, stick a polariser in front of the lens, pack a tripod and give it a go.
Obviously if you're looking for sunlight filtering through the leaves then disregard the above!
Apologies if I'm teaching my grandma to suck eggs!
All the best,