This is one of the hardest things to do well
haha, hardly. Ever had to get rid of solid white glasses flare?
I do contract retouching for a couple of local studios who apparently aren't that great. Taking people from one photo and putting them into another isn't that hard.
The part about the hair-
I usually put the most detail in the selection of the hair area, selecting more pixels than necessary, and after I paste them into the new background, go in with a low opacity, soft eraser and fine tune the hair. A bit of smudging with low pressure, and it's done. Transparency is the best thing you can do to the edges of people's hair when doing this kind of thing. Gotta let just a hint of the background show through on the very very edge of their hair (unless they're bald, in which case it's not a problem to begin with!)
Overall, my tool for clipping people out is the polygonal lasso tool, by holding alt/option in regular lasso mode. That way I can quickly switch between the two. I just zoom in pretty close (depends on how important the job is, but typically about 150%), and start making points. Always make more points than you think you'll need, because otherwise it will inevitably come out looking like you used scissors instead of magic (aka Photoshop).