World 1-2, right after the checkpoint flag, there are three koopa troopas and three coin blocks. Get on the first long coin block which is on the ground, then simply walk down onto the koopa troopa below when it moves right below the coin block. With a bit of timing and adjusting your position, you'll be bouncing off it repeatedly and start getting 1-ups. If you mess up and die, simply restart from the checkpoint. You can get an unlimited amount of lives in World 1-2, right after the checkpoint flag. There will be 3 Koopa Troopas and three coin blocks. Get on the first long coin block, which is on the ground. Whenever the Koopa Troopa moves right below the coin block, simply drop down on it and if done right, you'll keep jumping on it repeatedly for a lot of 1-Ups. It may take you a little adjustment to get it right.