Ok, this was a significant problem for me as well. Here's what you do. First, NEVER let your food need drop below 51%.
Second, when eating, pay attention to how many green arrows the food item has. That is an indication of how much it restores your food bar. As long as you are already at at least 51% full, any food that gives you more than you need will contribute to increased plumpness. The 'cheap & easy' option is always choose Cheeseburgers, they have the greatest addition per serving of any food in the game.
If, for some reason, you have allowed your food need to drop below 51%, first eat something that brings it back up above 51% before eating the item that takes it over full. If you are in one of the cafeterias, select multiple food items before 'finishing'.
Following these guidelines rigorously (particularly the 'never let your food need drop below 51%') will bring your plumpness back up. Note, 'never' means even while asleep, always make certain you are full or nearly full before going to sleep.
Edit: The final thing to be aware of, 2% is about the cap you will see for daily increase or decrease & that is only if you have gone to particularly extreme lengths. Regaining plumpness doesn't happen fast.