Get 50 or 99 Ultra Balls the go into some grass near-by and battle a pokemon e.g Nincada, then throw an Ultra ball at it, when it bounces on the floor 1st time Press B then when it bouncs the secon time press down.

WARNING: The cheat must be exactly on time or it will not work. And on Powerful pokemon such as rayquaza, groudon, kyogre or the regis it will not work 1st time.