You always can tell when it's a new season of Supernatural because Sam has a new do. His hair has changed a lot through the years and has been a major cause for debate for many fans. Are you a purist and like the shaggy mop of seasons 1-3 or the more cleaned up coif of the later seasons?
The topic has come up on chat boards, blogs and even at conventions. Sam, Dean, the producers and anyone else involved with the show have weighed in on the many hairstyles of Sam Winchester. The first three seasons the hair slowly became shaggier and shaggier. I guess going from a university student to professional monster hunter will keep you from regular check-ins with the barber.
The hair covered his ears and his forehead, which many claim is actually a five head, but once season three started, suddenly the hair was much more manicured. His ears were visible and so was his massive forehead.
This could have been a way to show a more mature look or to possible make him look more evil as he dealt with his new powers. The audience that originally started watching four years ago was older and they wanted Sam to stop being that naive college student and look like a real man. The season six hair was much more cleaner. Season five was practically a mullet, so they toned it down a a bit. I can only imagine that they have for the next season.