The few people I have taught to cast I have them visualize punching their thumb (assuming you cast with your thumb on top of the cork) towards the sky when stopping the rod on the backcast. This seems to help get them to stop the rod high.
I also like to get beginners to spend a lot of time just practicing the backcast and nothing else. This it what I spend the majority of my time on when I am practicing and it has really helped improve my distance. When I first started casting I had no idea how important the backcast is to being a successful caster. Every newbie should have this point driven home before they even pick up a rod.