If the Pokemon Game you are playing is Red or Yellow, then catch Mewtwo with it. If the Pokemon Game you are playing is Crystal, then clone your master ball and catch Suicune, Raikou, Entei, Ho-oh and Lugia.
You can get 4 master balls in pokemon crystal. 1st 2nd & 3rd: AFTER you defeat the gym leaders of Johto find prof.oak & he will give you 3 master balls! 4th: AFTER you beat the elete 4 and the champion you go to prof.elm and he will give you 1 master ball!
Have a Pokemon hold a master ball and enter a double battle with dtto and the Pokemon that's holding the master ball and lastly have ditto transform into the Pokemon that is holding the master ball and you have two master balls.