Just as we are all born being right handed or left, every person who sits on a horse has a " handed-ness". There is a tendency to sit more to a particular side of the saddle and a litany of associated ills that go along with that. Lets take an example of the person who sits too much to the left. This person will also be collapsing their right waist and lengthening the left. They will most likely be twisting their shoulders to the right, hiking up the right heal and be jutting the left leg forward to brace themselves from falling off that direction.
The definition of straightness in the horse (according to the USDF manual) is as follows, "A horse is straight when the hind legs follow the path of the front legs, on both straight lines and on bending lines, and the body follows the line of travel. Straightness allows the horse to channel its impulsion directly toward its center of balance and allows the rider's hand aids to have a connection to the hind end." Without straightness it is impossible to train the horse to have that connection and therefor be on the aids. A CROOKED RIDER CAN NEVER MAKE A STRAIGHT HORSE ! So unless a rider is willing to get down to the nitty gritty and address their basic straightness they will find there communication with the horse blocked from the get go.
As a rider, I know that if I want shoulder in or half pass or what ever I just think it and it happens. Oh yes, once it starts to happen you can deal with more of this or less of that. The point is, I put my body in is certain place and the horse follows it with theirs. Now if this body of the rider is crooked the horse will never get the message and it will never be easy. The struggle will ensue when the rider tries to counter act the messages that their crooked body is giving the horse. I know that this is the boring part for both rider and instructor. As instructors it is much more fun to teach " the movements" than stand there and hold a lunge line for 45 minutes while trying to get the student to straighten out their crooked body. This is so important for every student to work at this level to learn how to control the horse with the weight aids of their straightened body and not just the reins.
As an addition to this oh so important lunge work, I suggest Pilates with an emphasis on how to find your core and use it to stabilize your body so it can interact in a calm and relaxed way with your horse. Here is the NW we are lucky to have Beth Glosen (new book The Riding Doctor) who has made this her specialty to help riders in exactly this way. I am sure there are others out there. Take the time to check out the possibilities. Your horse will love you for it more than 10,000 bags of carrots!