This secret was first named “The Goomba Stomp Technique" by the Official Super Mario Bros. 3 Players Guide. This can be accomplished one of two ways. The cliché easy way or the hard way.
The Easy Way (Extremely Easy)
Locate a P-Wing and equip it. Then enter World 1-2 or the World 2 Fortress. Find the spot in the level with endless enemies either Goombas (World 1-2) or Dry Bones (World 2 Fortress). Start stomping these enemies and on the 9th enemy you hit in mid flight you will receive a 1-up. With unlimited flight the objective of earning 1-ups becomes pointless. Here is the more challenging method.
The Hard Way
This depends on how much you have played Mario Bros. 3. This so called Hard Way could be child's play for you. It is the exact same idea previously mentioned with a Raccoon Suit or Tanooki Suit instead of a P-Wing. The only difference between the Raccoon and Tanooki suit is that Mario can turn into an invincible statue as an ability. This requires more timing and coordination on the players part to hit the enemies correctly.
One question comes up for those of you attempting to try this secret. How am I suppose to get a P-Wing for World 1-2 since it's not available until World 1-4 earliest? The World 1-4 P-Wing is through the White Mushroom House coin challenge. An idea without using a Game Genie or game hack is by beating the correct Bowser to receive an inventory filled with P-Wings. The correct Bowser is through the very top door after the Lava/Falling Piece main area.
For an 8-bit game it is more than meets the eye. There are secrets rarely known of such as a completely Harmless Bowser for a final boss, a White Pirate Ship, and more. Enjoy trying this secret and many others in possibly the greatest game of all time.
Super Mario Bros. 3 at http://originalnintendo.net/mario-bros-3-online