Instructions
Place the Pokemon whose exp. you want to raise at the start of your lineup. This makes it automatically participate in every battle -- even if you switch it out for a different Pokemon right away, it still earns exp. points at the end.
Give the Pokemon a lucky egg to hold -- these increase the exp. points it earns in battle.
Coordinate with a friend to trade your Pokemon and train each other's for a period of time. A Pokemon that is not with its original owner gains exp. points faster, so make arrangements to trade, train, and trade back.
Feed your Pokemon rare candy when you come across it. These are, as the name implies, hard to come by -- as each one increases your Pokemon's exp. level by one, though, they make leveling up to 100 easier.
Battle wild Pokemon, particularly those that yield high amounts of exp. points. For example, in "Pokemon Black and White," fighting wild Audino gives you relatively high amounts of experience. Look for them in shaking patches of grass.