On level 6, Hazy Maze Cave, go into the hazy maze as any character except yoshi (preferably wario since he can become metal) and navigate until you find a red brick block, inside will be a mushroom. Grab that mushroom and go around the corner. There will be 2 moles, (i forgot their real names), keep running in circles over the holes in the ground that they pop out of and you will repeatedly kill them. After killing 10 you will start gaining lives, continue this until the mushroom runs out. You should have 40-60 extra lives now.