SNES: Codes For:Yoshi's IslandYoshi's Island  

Infinite lives 
but you have to get to stage 4, level 1 first. Near the beginning of the level 
you'll find a cluster of green pipes. Kill the plants with your eggs then go to 
the first green pipe that produces purple critters. Eat them until you have your 
maximum amount of eggs, then eat a red Koopa shell and jump onto the second 
green pipe in the group. Cough the shell back up so that it bounces between the 
second and first pipes. Shoot one of your eggs into the air. The purple critters 
will begin pouring out the first pipe straight into the path of your shell and 
you'll be clocking up the lives. 
Practice on the bonus levels 
Go to the level select screen, and press X, X, Y, B, and A, while holding down 
select..