At the main menu hold down the left shoulder button and type in the following codes. You will here a sound when you type in the code correctly. To disable the cheat type it in again.

Begin World Tour mode with $1,000,000:
Up, Down, Left, Down, Triangle, Triangle, Triangle

Earn $2000 every week in World Tour mode:
Up, Down, Right, Down, Triangle, Square, Triangle

Unlock Sepia mode:
Up, Down, Left, Right, Left, Left, Left

Unlock All Racquets and Clothing:
Right, Left, Right, Right, Up, Up, Up

Unlock All Stadiums:
Up, Down, Left, Right, Square, Square, Square

Unlock the players King and Queen:
Up, Down, Up, Down, Square, Triangle, Square

By the way is there a cheat for maxing out your character?