Are you using treats? Our trainer at petsmart says to hold the treat just above their head (make certain they see the treat) this is to effectively make them sit. Then when their bottom hits the ground, only then to say"sit", as you give the treat followed w/good dog (boy/girl). She alos said to not put your hand on their rear to force a sit. I've seen it work w/larger breeds very well, but so far Ruby is not a treat hound, which is making it tougher, but I'm hoping with work she will get it
I'm sure others out there have different methods, maybe one that will work better than this?