The best places to train is the stadiums in Nimbassa City because the athletes get stronger as you go through the game. There are more trainers at the two stadiums in B/W games than the in B/W2 games, so the best results for your Prinplup is to trade it back and forth, if you have 2 DS's, a B/W, and patience to spare. Since you say that your Prinplup is only 29, give it a lucky egg or an exp. share and switch out if the athlete's Pokemon are stronger.
Note that the people in the two stadiums change daily