Or not training them at all since there's an option to automatically make them Level 100 for the duration of the battle.
Honestly, this is the best option. Since you can force all Pokemon in the battle to level 100 and it doesn't squash stats the way "Level 50 All" would, there's really no reason to level an entire team up to 100 anymore. Just EV train your team, evolve them, teach them all the attacks you'd like and you're good to go.
Still, if you really want level 100 and don't have rare candies to abuse, track down a wild Chansey that's holding a Lucky Egg. Easiest way to do this is to
chain Chansey with the PokeRadar and lead with a Pokemon that has the Frisk ability (so Shuppet, Banette, or Stantler).
Once you have that, either battle the E4 a lot, battle on the post-Elite 4 island, or - better still - trade your Pokemon to another D/P game in order to gain boosted EXP. Obviously you're only going to be able to do that if you have two DS's/two copies of the game or have a friend willing to lend you theirs/raise the Pokemon for you, but it is a potential option.
Hope this helps. And please, for anyone else who replies to this thread, pay attention to where gimmepienow mentioned "without cheats". Replies about cloning rare candies or hacking them are therefore useless and unhelpful >>;