On any world go to a mushroom house and select an item, if you can get to a life spade, do that to, if not, do it later. Then after you got all that, save and QUIT, it will not work if you save and continue. Then when you turn the game back on (you do not need to shut off the snes) you will have the item, and the mushroom house will be back, repeat as wanted, if you didn't do the spades, do the same thing. I am not sure whether you can do this on the nes because you have to save and quit.I know this will work for you because the game was originally programed to reset levels when the game was shut off, but now you can save in the middle of a world, so the mushroom houses reappear and the spades become reusable, but since you saved, you keep those items.
Do you like a good challenge? If so, you are really going to like this. If you make it to World 8-4, after you defeat Bowser, the princess will challenge you to a more difficult game play. Accept, and you'll start off with black Koopas instead of those runts, Goombas.