You could use many different methods to level. The item Lucky Egg increases your Exp. Points, and trading your Pokemon to another game increases as well. Fighting or rematching trainers is also the most simple way. If you're playing Black/White, fighting wild Audino in rustling grass, Basculin in the water or other evolved Pokemon in the wild gives you a lot of exp. Rare Candy is the easiest way, but they're rare to find.
you train them
Train them............
if you want to level up your pokemon faster. give the pokemon you want to level up an exp share but it at the bottom of your pokemon list and battle and beat the elite4 in succession over and over until it is at the level you want it to be.
no
lucky egg
take a weak pokemon to the elite four
aerodactyl. It's like the fastest Pokemon in the game.
not a cheat in my opinion. you trade your Pokemon for it
YOU HAVE TO GET ALOT OF rare candy.
Its the same either way.
Try using Rare Candies for easy levelling. :)
if your Pokemon level ups, its happiness will increase automatically. but if you give your Pokemon a soothe bell, its happiness will increase faster, and it will evolve faster (by certain Pokemon). also return will do more damage