What can I do to prevent bladder stones?

Urinate. Bladder stones are caused by one's inability to empty the bladder. The best way to prevent bladder stones is to keep your bladder empty, many times requiring medication or surgery to do so.
Drink abundantly. Bladder stones are often seen in the setting of obstruction of the lower urinary tract, often from prostate enlargement with incomplete bladder emptying, so keep urine dilute with plenty of water and make an effort to void to completion. Stones can also be seen in the setting of neurogenic bladder and the presence of a urinary catheter and once again, dilution is important.