Buy Ryan Hall's dvd on Back Attacks if you can.
IMO the upper body control is more important than the hooks for not losing the back.
You want your spine to stay in alignment with theirs. A good seatbelt or kimura/seatbelt grip is important, at least for starters. Try to "melt" onto their back like a turtle shell. You want your shoulders behind their shoulders, and you really want to tuck your head so that you're both basically ear to ear. The muscles in between your shoulder blades will be sore when you practice holding your seatbelt at first and staying tight.
If they can rotate their back/spine off line of yours, then they will start to escape.
The lower body control will come but at first you want them to be able to try and move around or roll around while you stay stuck to their back with your upper body control.
That's very conceptual but I hope it helps some.
