Buy any number of condiments and place the food item you plan to use them with at the bottom of the character holding the condiments's inventory. Make sure it's at the bottom of their inventory. While in-battle (and in-battle only!), eat the food. The condiment will disappear as normal, but the food itself will remain! You can conserve good food items this way.
An obvious perk to this is the Rock Candy item which ups your stats. Buy a lot of Sugar Packets (they double the stat growth) and use the Rock Candy over and over to pump up your stats! Note that any stat over 255 will cycle back to 0, so be careful. HP and PP can continue into the thousands, though the bar will be graphically glitched and no scrolling will take place if they receive a hit.
Submitted by Artemis251.