Most all of the background changes that I do utilize a layer mask to hide or reveal components, instead of cutting out a selection of the image. This way you can work on problem areas with the method that works best. For me, this often ends up by painting free hand on the mask. I may make a rough selection to create the initial mask, but I seldom find a selection method that will work well for all parts of an image.

forum discussion: http://forums.dpreview.com/forums/read.asp?forum=1006&message=30867906
original image by l_d_allan: