I agree with flamingo - cheap saddles do not help in the slightest. My position goes to pot in some saddles, yet is much better in others.

I did just learn another trick for keeping the leg back and underneath you, though. Mine tends to slip forward when I'm not paying attention and this way might be a good method to get the to 'get' where their leg shoudl be. Have them take their feet out of the stirrups and remind them to sit up STRAIGHT throughout this whole exercise. If they start to tip forwards, it's cheating. Then, get them to bring one heel at a time right up behind them, eventually using their hand to pull it right up to touch their bum - all thewhile sitting up straight and looking forward, as you do when riding. This should bring their thigh, which is the part most people let slip forward, into a further back position and open up their hip slightly. Then, ask them to let go of their ankle and let their leg fall SLOWLY back down but ONLY from the knee. The upper leg should, as far as possible, stay in the same position as when they had their heel pulled up to their bum.

And, of course, no-stirrup work in general. It's a killer but it works.