(c) John Corney 2008
In creating this image you'll see that I first of all cropped out a lot of the distracting background so as to focus just on the agave.
I then selected the remaining background and darkened it. I found that the new Quick Selection Tool that comes with Photoshop CS3 worked perfectly in this case since there was a nicely defined border along the edge of the agave; the tool was able to find this edge easily.
I saved off the selection of the agave(Select>Save Selection) in case I needed to work with the selection again and also for what I was going to do in the following images. I wanted to essentially make the remaining pieces of background between the agave "leaves" black, but if I were to just simply fill those areas with black it could look unnatural, so first of all I actually blurred these areas using Lens Blur and then filled them with black and lowered the opacity down to something like 80%. This way these parts of the photo don't just look like they were cut out and blend into the background.
I did some adjustments to levels and layers as I always do, and then I made a copy of the merged layers and in this case changed its blending mode to Luminosity, also adjusting the opacity and fill levels a little. Essentially they are the steps I used in the "digital darkroom" in creating this photo.
The original photo of the agave.
Agave Study #2
Agave Study #3