In every game, I keep the Pokemon I train. I don't see the point of raising a Pokemon up to a certain point, only to drop it. Usually if I want to change my team, I actually start all over. The games are so fun that its not really an issue to play again from scratch, experiencing a new roster.
Ever since RS/FRLG/E, and into DPPt/HGSS, and now BW, I only catch the Pokemon I plan on raising. I'm loving how you don't have to catch the Pokemon to complete your Pokedex, so I don't even catch anything unless I plan on actually using it.
Anyway, here's my White team.
Zekrom
Samurott
Darmanitan
Excadrill
Galvantula
Bravairy
And my PC Boxes? EMPTY!!!!!!!! Every Pokemon I caught, I'm carrying!