I'm proud to say I've done some good scar-work, so I can help you there. The first thing to keep in mind (but something you shouldn't worry too much about) is that the scars follow the contour of the face. If you spread the surface of the skin onto a flat plane, the scar will basically form a straight line (there are millions of exceptions, but usually the most commonly made scars will be straight). So when you sort of papier-mache the skin onto the curves of the face, you have to keep in mind where the scar will go and how perspective affects it.
Never make the scar "smooth" or it'll look like a skinny banana or face-paint. Make the edges of the scar very slightly jagged or rough to add realism and the appearance that the skin actually broke when the scar was formed.
Where scars exist, no fur grows. In relatively undetailed drawings, it won't matter, but with a really in-depth and complex piece (where the fur is highly detailed) you'll want to show how the fur ends, how the scar begins, and how the fur hangs/doesn't hang over the scar.
Scars come in many shapes, sizes, and widths, so go crazy.
EDIT: When I said "never" draw scars a certain way, I meant "usually don't." Art is never defined by any specific rule-set or procedure. Even though scars drawn realistically typically look better in most people's opinion, there are always reasons to defy convention and develop a unique style. Just look at Kenshin:
http://images.animelab.com/gallery/Rurouni_Kenshin/Images/kenshin329.jpgHopefully, I'll get some examples of scars that I've drawn (once I figure out how to work my scanner
)
