How to Level up Pokémon in Pokémon Black and White

Method 1

Battle Wild Pokémon or trainers around your Pokémon's level or higher. Audino is a good Wild Pokémon to battle, since it's a commonly-found Pokémon that gives a lot of experience. You can find one in shaking grass.

Give your Pokémon a Lucky Egg, which you receive from Professor Juniper at Chargestone Cave.

Give two other Pokémon Exp. Share. You can find Exp. Share in Castelia City and in Icirrus City.

Try to get the Pokérus. Pokérus is a Pokémon virus, which causes your Pokémon to grow better. There is no guaranteed way to get it, but you can catch it from wild Pokémon on special occasions.

Try trading your Pokémon to a friend, then get them to trade back. This will cause the Pokémon to level up faster.

Rustling Grass

After you get the first badge, you have access to rustling grass. Most times it will be Audino but on small occasions there will be others.

Fight these special Pokémon. This will give you more experience than the other Pokémon you run into in the tall grass.

Keep trying. Rustling grass will come up every once in a while. The best thing to do is walk/run next to grass, but not in it, until the grass moves.