Your span is a function of the size of your hand. I cannot advise strongly enough against doing anything that tries to increase your span as it could lead to serious permanent injury. I can't stress this enough...a small tear in a ligament in your hand or forearm can lead to chronic pain, loss of mobility, loss of sensation, or worse. If you think you're limited by the size of your hand, that pales in comparison to trying to work around a traumatic hand injury. Just because you can't reach a 12th the way Rachmaninoff did doesn't mean you can't give an inspired and moving performance of his compositions. I advise working with a teacher who can help you work around any physical limitations you may think you have.
Does anyone know any good and effective exercises to increase handspan in the long run?
Tie your little finger to a stable door and tie your thumb to a horse. Then kick the horse.
There is no safe way of doing it.