Want to train your Pokemon? Want to do it fast? This is the guide for you! This will give some tips on putting on the levels.
The Elite Four
This is the overall fastest way to train, though certainly not the easiest one. Essentially, you give a weak Pokemon an Exp. Share, and put a strong one (Rayquaza and Mewtwo) first out. This can be used any where, but the quickest is to go through the E4 over and over.
The Vs. Seeker
This only works on Fire Red and Leaf Green. Go to One Island and run up to the beach where the Black Belts are. Run into the cave, and go to the center of the pool to heal your pokemon and head out. This nearly recharges the Vs. Seeker. Then use it to battle the Black Belts and Crush Girl. Reapeat. If you haven't gotten there yet, try Nugget Bridge, Cycling Road (towards the top), and Route 10.
The Secret Bases
This one only works in Ruby, Sapphire, and Emerald, and you need two cartridges, two Gameboys, and a link cable. Both games need Secret Bases. Basically, you get six easy to beat Pokemon, that give lots of Exp, at high levels. Then, just give it one move, that can't do damage.Some examples:
Gyrados/Milotic: Splash
Gengar: Nightmare-
Arcanine: Odor Slueth 
Magneton: Metal Sound
Ninjask: Harden- Anyone: Snore/Sleep Talk
Then, get all six "worthless" Pokemon in a party and mix records. Then, go to the other Secret Base and fight them every day. Very fast, very easy.
Wild Pokemon
Just go wherever you see fit and fight everything you see. You can find lots of neat Pokemon this way, so have plenty of Ultraballs.
The Day-Care
This is where you put some Pokemon in the Day-Care, as soon as you get there,and play as you normally do till you beat the game. Then, come back, trade them to another game for holding, and repeat.
