I started my mare using a 10 ft lead rope and getting her to yield her hindquarters to me on a small circle, gradually increasing distance. If you don't have enough pushing power to get her out on the circle, you can always begin by walking with her on the circle. I associate the "driving power" with the whip pointed at the hindquarters, and encouragement to stay out on the circle and not fall or turn in is usually accomplished by directing the whip at the shoulder. You may have to adjust the position of the whip as you deem necessary. If your horse turns in to face you often, it usually means you are in positioned too far in front of the driving line, nearer to the head. You want to be at least behind the point of the wither. Your whole body language has to be pointed in the direction you want to go in the beginning. You can even look slightly forward, or concentrate on her footfalls. This means shoulders forward, almost walking into the line. Keep your elbows nearer to your hips, as your hips harness all the pushing energy, and this will be evident in your walk and footfalls, so make sure you are balanced over your hips, shoulders up, knees slightly bent, and feet grounded. If you decide to pivot, make sure you cross your legs over one another and both are facing the direction you would like to go. Also, desensitization to the whip is great, and necessary, but be sure your horse knows what one truly feels like if you need to employ it. A sharp rap on the bum shows follow through, and horses are naturally associative, so in this instance, take advantage of that. It shows you are decisive enough to be a fair leader, and it will get you far more respect and save you plenty hassles down the road.
Although tradition will tell you to stand in that standard "pie" or "triangle" shape, it didn't quite work for me in the beginning as it does now, so maybe you too will have to work up to that.
