Hair… uhm… well, if you’re gonna add some, just try to find something as close to the real hair of the player you’re going to edit to match it up.
Be aware of the direction of the strands, and color correct where necessary.
Smudge the added hair and the original hair into each other to make it blend better.
In case of removing hair, I’d find a bald head that matches the pose of the original and smudge the original and the bald head into each other, again color correcting where needed.
Clone stamping is also quite a useful tool, just experiment.
Hope that helps. I’m sure, no, positive, that there are people out there far better at this than me. As you can see in this half-hearted edit(try to guess the celebrity used for the head):
EDIT: Also, I’m a firm believer in that the crappier you make something(in terms of realism), the funnier it becomes, so don’t worry. SF: Animated FTW!!! 