1. When there's a nicer ball out/better. I will buy it.
-Rich
2. Bowl till the coverstock cracks.
3. Bowl till it can't hook any more.
4. I have changed my style. Then I might buy a new ball. – Weight,
more aggressive or something.
5. My family/who is a bowling ball sellers or getting as a birthday
gift every year. I can upgrade my balls free/cheaper.
-blessed
6. Often getting old balls from my pals who doesn't wants them. I
will replug & use them. –Hangs around with rich or etc
pals.
7. Every time if I win in a league. I will upgrade my ball with the
prize money 
I will pick 2 & 4