Hit select during gameplay and press:

Up, Down, Left, Right, X, Y, A, B, Start, Start, X, Y, A, B

The developers symbol will appear after you have done so and a sfx will play. Press start and you will now be able to enter codes. The full cheat list as found by Eduard Beluha is as follows (copied from his website):

Select - Enter the Debug Warp Stage. There are two big rooms. Entrance to the second one at the top-right corner of the first one. The first one filled with doors leading to various game checkpoints. The second one consist of the doorsto the all ingame subrooms. [sp.] Right - Unpause with shake effect (not sure for what). B - Toggle Invincibility (complete disable collision test). A - Free Movement mode (be carefult, some edges of the sage may cause instant death). [sp.] X - Best Ending Y - Good Ending