Yup, Mica covered the most important part. I'm sure if you have a professional comes in here, there's very scientific procedures used and such. I'm an amateur, so I just keep fiddling with it until it looks right.
Few things you should do to make a fake look very real:
Make sure you get the head/body proportioned correctly
Make sure you get the brightness of the faked parts to match the rest of the scene
Fiddle with the colors of the layer to get that to match too
learn to use the stamping tool. It performs miracles.
For advanced users:
(my opinion only) avoid pics that have been circulating around the community for years. It takes away from the freshness of your art, and tears down that plausible aspect of the scene
use PS's advanced featured like light renders to match the layer up on the scene. If the scene's at a beach, you'll probably want to cause a bit of glare. If it's in a dark room, the opposite is needed
I like to blur and sharpen the layers as well, to have them match up with the scene I'm working with. TV vidcaps tend to be quite blurry, whereas magazines scans and the link will usually be high quality stuff. Almost impossible to go from blurry to sharp (when you're using extremes).
Hope this helped. I love this particular artform, and love to see new faces popping up on the boards with their fakes in tow.