Instructions
100 Free Men with Yoshi
Start Level 5 in World 2. This is the desert level with the giant peach caterpillars. Hit the question mark blocks as you run through the level until you find Yoshi. Jump on Yoshi and make sure not to lose him. If you do, you'll have to catch him or restart the level.
Ride Yoshi through the level until you reach an area where a Koopa on a cloud is throwing little red, spiked turtles at you. Make Yoshi eat one of the spiked turtles and keep it in his mouth. The main area the cloud Koopa hovers around should have a slight hill in the middle and a block on either end to keep items from falling off the edge. Spit the spiked turtle on the ground. Jump onto the question mark blocks above you to avoid hitting the turtle and accidentally killing him. He should bounce back and forth between the blocks on either end of the area.
Move between question mark blocks to avoid being hit by the spiked balls the Koopa throws. As the spiked balls fall to the ground, the spiked turtle will smash through them and destroy them. Each one it destroys earns you an extra life. Stand here long enough and you can earn 100 extra lives, or as many as you need. Move to avoid the falling spiked turtles the Koopa throws.
Classic Turtle Trick
Reach Level 4 in World 2 and look for a set of stairs with a turtle walking down it. The set of stairs is seven steps tall if you count the top as a step.
Jump on the turtle as he reaches the third step from the bottom. You need to time it so that you hit him just as he reaches the edge of the step. Land on the edge and Mario will bounce on the turtle once, knocking him back against the stairs, and when the turtle shell flies forward again Mario will automatically bounce on him again.
Leave the controller alone and Mario will keep bouncing on the turtle shell. After a while, you'll start getting extra lives and can leave Mario there until you've earned 100 lives. Keep in mind that this method of earning extra lives is hard. It takes precise timing and a little bit of luck to hit the turtle just right. Practice timing the jump and eventually you'll be able to earn extra lives any time you want.