How to Level Up in Pokémon Diamond and Pearl

Steps

Put two of the Pokémon you wish to level up in the daycare.

Put two level 70-100 Pokémon in your party.

Get four Pokémon that are weak and you want to level them up.

Give them Exp. share. If you don't have enough, then start by training one of them, and then switch every time you finish a battle. Easier target is Elite Four since you got two level 100 Pokémon.

Go to Stark Mountain or Turnback Cave with a level 100 Water type Pokémon and an exp. share on the Pokémon you are raising.

Eterna Forest is a good spot to train with Cheryl. You get more than 200 experience each battle and a full heal. It's a good spot to train for Gardenia.

Wayward Cave is a good spot to train with Mina. You get more than 200 experience each battle and a full heal. It's a good spot to train for Mina.

Alternate Method

Buy full restores, revives, max potions, etc., until you don`t have money or you can't buy more things.

Talk to the guy in front of the door to go to the elite four and beat your rival.

Go and try to beat Aaron. If you beat it beat Bertha and etc.

You can give an EXP. Share to your low level Pokémon and go to Victory Road and the Elite Four and keep battling to get your Pokémon's Level up.