there are a couple ways i mask things – sometimes with the lasso tool, but mostly with the pen tool. if you use the pen tool, once you’ve completed your path, in the path palette, select the icon to convert the path to a selection (load path as a selection). at this point you can choose to feather your selection – depending on the situation this feather will vary – I tend to feather .5px – 1px depending on the size of the original image. then create your mask on that layer by selecting the mask icon at the bottom of the layers palette.
LittleBlackBird.net
GraphicsDump.com