By stabbing a doll and fighting with the physical manifestation of your sword (otherwise known as ZanpakutÅ) Oh and also you have to have an out of body experience... Quite literally.
You ether A: Train your physical and mental bodies for hundreds of years at the soul academy, developing a relationship with your sword so great that it will lend you its power to achieve bankai........
Big question!!!!
I would say, to grow in life, but in 100 years from now all of us are going to be death, so what's the point?
Then maybe, grow spiritually? But unless you believe in something...