I've been driving around in Just Cause 2 like a maniac at full speed but I very rarely get any stunt driver points. I'll occasionally get one - or, rarely, two - when I drive into a sharp incline that makes my car jump into the air, but it doesn't seem to work consistently. Very often I won't get anything. In 20+ hours of play I've only got around 15 points, so getting the 'Stunt Driver' achievement seems a long way off.
Are there specific types of stunt that are rewarded more? Do certain vehicles make it easier (fast ones, I guess)? Are there prime 'farming' spots?