So, you've beaten the Elite Four and finished your Pokédex. After that, it gets kind of boring. Not if you have a Data Link Cable. Battling against your friends is a great way to put some replay value into Pokémon. Here are some rules that I go by when training my Pokémon.
1) Your team should consist of a variety of different Pokémon. You will probably want to have the following on your team;
Position 1 - Electric
Position 2 - Fire
Position 3 - Grass
Position 4 - Water
Position 5 - Optional
Position 6 - Optional
Make sure the two optional Pokémon you pick are not of the same type as any other Pokémon on your team. Some things you may want to consider when picking your optional Pokémon are speed, fighting ability, and what the Pokémon will be effective against. You may want to refer to the Battle Chart when picking your Pokémon.
2) Raise your Pokémon's level's evenly. This is not important, but it will be handy to have an equal team. Your team will be easier to raise if all of your Pokémon have been raised equally.
3) When your Pokémon learn new moves, make sure you have at least one defensive move (Growl, Leer, or Recover). They will come in handy if your opponent is of equal level or strength.
4) Use stones only if you think it is necessary. Sometimes, if you use a stone too early, you will comprimise one strong point for another, it's up to you to decide which one is more important. For example, when you use a Thunder Stone on a Pikachu, you will lose Pikachu's advantages in speed for Raichu's more powerful attacks. Power is nothing if your Pokémon can't take hits (or avoid them).
5) Train your Pokémon against reasonable opponents. Make sure the Pokémon you are training is of a little bit higher level, and has an advantage over the Pokémon it is battling. For instance, train a level 30 Ivysaur against a level 25 Blastoise. This will assure your Pokémon a victory, and that it will be able to enter many more battles without being drained of energy.
6) Don't forget about items such as HP up, Carbos, Iron, and other energy enhancing items. If you need more power than your Pokémon has, there can come in handy. It will make a Pokémon stronger than it usually is at it's level.
7) When your Pokémon have reached a suitable level, you should take advantage of being able to re-face the Elite Four. Not only will your Pokémon earn tons of experience, but they can earn you some extra money!
If you have any tips on training your Pokémon to level 100, please send them to me so that I can use them on this section of the website. If you would like, I can give you recognition by putting your email address by the tip. This section will be constantly expanding, so look for more tips periodically.