When you get a power-up mushroom, start plowing down enemies, signs, etc. A number will appear after every object you hit. After the number "8" appears, you will earn a 1-Up for each enemy hit. Additionally, when you enter the castle, go to your right. There will be a door with a "3" on it. Enter it. There will be a painting with bubbles inside. Go to your left. There will be a raised box up in the wall. Jump inside and there should be a 1-Up Mushroom.