I find that a big part of the solution is arching to spread out your opponent's body before reaching for the choke. Drysdale demonstrates this really well in his DVD's. You could also just choke over their jaw, but it's best to learn the optimum way of choking across the neck.
I'm having the same trouble, but as others have said, it takes a while to get good at RNCs. This is really helpful though, in case you haven't seen it:
Put your wrist under their nose and pull as hard as you can. Guarantee to raise their head and leave their neck WIDE open.