Read up on this move a lot due to MG's material out on it. biggest detail I learned was using my ribs/lat to redirect the chin away from me to tighten the choke. Became my goto from kob and side control. I was surprised how great this sub was and this is all in the gi as well.
One thing that I can't seem to address is when they turn their body into me/ they face their body towards me. It seems to relieve the choke a bit and I end up ditching the move to side control. I tried posting my right arm on their hip/belt to keep them steady but just cant seem to stop it.
If you are having trouble on the entry, Marcelo and Kesting troubleshoot the position a lot on this video, including what happens if they turn their head into you:
http://www.youtube.com/watch?v=RkFHJHC58qc
^^^ I think that video will solve your problem. If not...
If you are already in position, and they end up turning into you, you can convert the north south into another choke, like a darce or a guillotine. This video from Jason Scully shows one such transition:
http://www.youtube.com/watch?v=WQAZTikKu7c
If they're turned in enough to block the choke, but not far enough that you can sneak a guillotine in, sometimes you can swap arms (i.e. to N/S choke on the other side) and catch them off guard.
Grab the leg/pants and turn their knees away. It is very hard to completely turn in while your legs are facing the opposite direction.
You can switch to NS Kimura very easily from there as well :) 
That second video and pointyshiny good tips. I actually havent thought to turn it into another choke haha. I typically look for farside armlock when they fully turn into me but will have a go.
I'd really like to know if there is a way to salvage that choke though. The first video is among a few that I used to start finishing with this move but this last bit has snagged me for awhile now. Reset? use the loss to chain another sub?
sub
sub
Got to watch the vid that robhustle posted, but I kind of like when they have their chin turned towards you a little. You can get your lat muscle under the chin, then as you do a big step back with your bottom leg and go chest down, your lat pulls his chin up and your ribcage pushes against the side of his jaw to turn his head to pont straight up.... Never had much luck with the N/S until I started doing this.
Pushing the knees across, heavy on the lat pressure, also sliding the mat leg thru while posting with the other leg to help keep them flat. One thing inn particular that helped me with this choke it's that I literally "looked down" as I was transitioning to n/s. Once my head was low enough, I just traced across their body with my head till I
was in proper position