I also use Phitoshop actions a lot for this kind of thing, saving the mask in the alpha channel.
Eg. Pad image with white, magic wand select top left corner, add to alpha and remove padding. Masks any area of white (intended for sky) that meets the edge of the image. Handy for when branches/building create multiple regions of sky.
Perform colour range selection based on white, either on while image or within a selection and add to alpha. Useful for masking all areas of white sky between branches leaves of trees.