Always Get Desired Pokemon:

Go to training mode and continuously select 'pokeball' from the menu. Whilst throwing them, look out for your favourite pokemon to appear, and STOP on that one. After it has done its move, go to 1-p adventure or classic mode, and the first pokeball you open will be the one preveously found on training. After this, the rest of the pokeballs you open are random. A good trick to use in this way is to do it with mew, because you get bonus points for seeing her.