the best mask is the one you create yourself.
usually the path tool in photoshop would be used to create the shape. the saturation filter would indeed be a good choice once you have your selected area as oddy mentions.
trying to use magic wand in a picture above however wouldnt produce a good result for your selection tool. the range of pixels the magic wand would grab would spill into the undesired side around the babies hand.
this path technique is usually referred as deep etching, a common practice used in all those magazines / newspapers you read with products on white backgrounds etc..