Do you happen to have an example source image to share?
Here's a tutorial that uses the blue channel for a fairly clean sky.
It generally works for busier backgrounds too. The trick is really finding the channel (or combination of channels) that extracts the contrast between the leaves and background.
This is a 33Meg Quicktime video tutorial:
http://www.jushhome.com/nick/tutorial/m ... ut_v01.mov
If you post an example image, maybe a few people could comment with specific ideas for your particular challenge.
Hope this helps.