These methods have been named the best ways to get exp (without cheating) by many of my Pokemon-playing peers:
Method 1 (if you have the TM for Thief)
1. Find a Banette and catch it.
2. It will have one of 2 abilities, Insomnia (prevents sleep) or Frisk (reveals enemy held item)
3. If it knows Frisk, teach it the move Thief, this move steals the enemy item.
4. Go Clefairy hunting. Clefairies occasionally hold the Lucky Egg item. It increases exp. use Thief to steal it. then either beat the Clefairy or run. Your choice.
5. Trade the pokemon you want to level up to a friend then trade back. This gives it the "Gained a boosted *****EXP!" thingy after a battle.
6. Give it the Lucky Egg.
7. GO BATTLE CRAZY.

METHOD 2 (if you dont have the TM for thief)
1. Get a Banette that has the Ability Frisk.
2. Go Clefairy hunting.
3. Clefairies occasionally carry the Lucky Egg item. Catch the Clefairy if is holding the Egg.
4. Catch the Clefairy
5. Take the Egg.
6. Release the Clefairy.

*NOTE: Having a Banette that knows Frisk AND Thief is extremely useful. It is a good way to get rare berries or items held by wild pokemon.

From- Trainer# 61742, 3 Star Trainer