Infinite Credits
Enter the following code at the Title Screen:

Hold Select then Hold Start, release Select.

Then at the Options Screen Hold L and Press Up + X - Left + Y - Down + B - Right + A then release L.

Doing this correctly will give you infinite credits

Extra Mode
On the Title screen, hold Select and press Start to access the Options screen. Then, press and hold R and enter the following: simultaneously press Up and X , Then Right and A, then Down and B and finally Left and Y. If you enter this correctly, the screen will switch to the Extra Play Mode. Use the Control Pad to change options.