Inside the Castelia City Pokémon Center head upstairs and you will spot a shady looking man dressed completely in black. In actual fact he will give you items depending on how many trades you have completed. This is calculated by how many different ID numbers all of your Pokémon have, and if you reach the magical number 50, you will receive a very special item indeed:

Inside the Icirrus City Pokémon Fan Club, speak to the President and he will ask you to show him a Pokémon. You can show him any Pokémon as it is not the species but the level he is actually interested in. When you later return and show him the same Pokémon he will give you an item depending on how many levels the Pokémon has grown: