A variation on this technique (and my preferred method) is not to use the whole second photo for the replacement-face layer. Instead, use the lasso tool to grab a small chunk of the image containing the replacement face and head, and copy just that small section to a new layer. Get the whole head and hair, neck and collar, and a couple inches of "headroom" around the head.
If your two images don't line up - you didn't use a tripod, or people were moving around a fair amount - it'll be easier to line up the replacement by transform/adjust just that new tiny layer, instead of trying to transform the full-size frame to make just that one head line up.
Temporarily set the new face layer to "difference" mode or some other oddball blending mode, this lets you can see through and line up the important part of the replacement head (often the collar). Then transform it back, add your layer mask, and use black/white brushes to do the replacement.
Before you wrap it up, Alt+Click on the mask in the layer thumbnail, this will show just the mask on the screen - so you can see if you missed brushing in anywhere.