When composing layers and blending them together in Photoshop, you sometimes need to vary opacity in certain areas. Obviously - a layer mask is ideal for this. There is a number of techniques you can use on layer masks to make cutouts easier and quicker.
Gradients come in very useful to create a smooth blend, selections can also be very useful too to isolate an area. To smooth the transition from a selected area remember to feather the selection before applying your gradient.
To get varying opacity you can paint inside the area with a brush, and then brush back over the area with the history brush set to low opacity.
Sometimes there is good potential mask detail in a channel or the actual image itself, this is especially true for the always tough hair cutout job! Literally copy the channel or whole image, and paste it into the layer mask. Just with a quick tweek of the levels you can get a great result!
Remember you can apply filters to layer masks too, like a simple, light Gausian Blur to soften it, or even Motion Blur for more dramatic effects!
BONUS TIP
Remember that you can copy your layer mask to a layer set (or any other layer) very easily by simply clicking on the layer you want to copy the mask to, and then clicking and dragging the layer mask of the source layer down onto the new layer mask icon! This is very useful if you want to duplicate a layer mask onto a layer set so the one mask effects multiple layers.