it's hard to sit out a spin (having been set on the ground 5 times by a spin in the last 3 years) but , what usually causes the most porblems is if you are braced in your stirrups, with them sticking out in front of you, you have a hard place from which the rest of your body gets "launched " off of. The locked knee and boot braced into the stirrup makes your whole upper body react to a spin by actaully pivoting off that rigid spot and swinging, like the "crack the whip" effect. Really builds up centrifugal forces and your upper body just falls forward and off the horse.
If you are balancing right over the center of your body, with you legs down below you , you will not be flung out onto the circle of the swing so badly, less centrifugal force and you'll tend to pivot WITH your horse, right around the very center of his body.
So, one thing to work on is NOT bracind your knee, NOT pushing hard into the stirrup, and sitting upstraight, never slouching.