I used to use Corel Knockout 2 for this type of thing, but sadly it seems to have been ignored/abandoned by Corel - they haven't updated it in years and the 'current' version doesn't work at all with recent versions of Photoshop. I wish they would bring it up to date as it was incredibly easy to use.
Anyway, on the plus-side, there's a book I'm planning to buy which is recommended quite often as the best book on masking techniques:
Photoshop Masking and Compositing by Katrin Eismann
VinnyP - I've read somewhere that chromakey style backgrounds are not necessary for doing digital backgrounds for stills (but they're very popular for automated processing used for video). Do you find they help with stills too, or do you use yours for video? Do you use specialist software to extract the subject from the image?