When you're photographing a group of people, it's rare to have a shot where everyone looks good, even if you take a couple of spares for "insurance". Using copy and paste to replace one "bad" face in an otherwise OK shot can be fiddly and rarely works well, but Photoshop blogger Helen Bradley has an effective alternative method.
Bradley's technique (which you could adapt in any layer-based image editing program) works provided the two shots have essentially the same poses and background, which is often the case if you've asked people to sit for a shot. It involves layering the two photos and then painting in a mask over the face you want to replace. It's not a one-click solution, but it allows for simpler tweaking than trying to select and match a face from two separate images.
Hit Helen's post for the full procedure. If you're not confident with Photoshop, check out our Lifehacker Night school series on mastering image editing.
Photoshop - Replace a Face [Project Woman]