First of all, if you never watched Cesar Gracie BJJ instructional, you don't know what are missing. It is one of the most detailed and complete instructionals out there. I really recommend buying it.
Anyway, on the beginning of volume 3, he describes the proper side-control positioning, which I would like to share (just 2 minutes long):
Remember, before you go to a submission, you need to make sure you hold the position (and gain the points). I specially like his suggestions regarding the foot movement and far-arm control.