This is pretty much just like the Rock Candy glitch. Just make sure whatever healing item you use is at the end of the list in your inventory. Fill your inventory with condiments and use the item. One of the condiments will be used, but your healing item will not be.
Salt Packets and Brain Food Lunches work really well. They will give you Maximum HP!
By checking the drawers in Threed's hotel, you can find an Insignificant Item. It may seem...well...insignificant...but if you give it to the man in Twoson's hospital, you will be traded for a Magic Truffle! It's a sweet deal!
Usually, you can't exit this place, which is right before the final boss. This means you can't go back to stock up on items or anything. But in the Japenese copy, Mother 2, you can exit. All you have to do is have an Exit Mouse, then while you're there, use it to exit. You can always go back. This only works in Mother, not Earthbound.
Okay, this is really easy, and totally, or more. Worth it. All you have to do is simply clear a character's inventory so all that you have is a single Rock Candy. Then fill the inventory with packets of sugar. Use the Rock Candy in battle and watch your character's stats raise extremely. You can do it until you run out of Rock Candies.
So after you beat the game, you're free to walk around and see people one last time. But what's different? No enemies are around anymore. If you want to have one last battle, there is a way.
Back when you were in the museum at Summers, remember when the Shatter Men attacked? Well, it's possible to only fight one of them, and leave the other one there untouched.
If you beat the game and go back to the museum, it's still there! But the cool part is, if you die by it, the game freezes! But that's not the best part! Turn back on your game, and the file is erased before you could even fully finish the game! Isn't that fun?
Near the beginning of the game, when you make it to Twoson, go to the hotel. There should be a man inside. Talk to him and he'll say something random, as if he's paranoid. Continue to talk to him and he'll eventually give you $50.